The difficult thing for me was finding an entrée, which turned out to be 12 across. Subsequently, progress was made, perhaps slowly, but definitely surely. Thanks to Brummie. Definitions are underlined in the clues.
Across
1 Monkeys: small groups of like-minded individuals (6)
SCAMPS : S(abbrev. for “small”) + CAMPS(groups of like-minded individuals who, say, support a certain doctrine, as in “the socialist camp”).
Defn: …, as used to describe mischievious children.
5 Top-notch detective capturing hearts (8)
CHAMPION : CAMPION(Albert, fictional detective in Margery Allingham’s series of books) containing(capturing) H(abbrev. for “hearts”, the suit in card games).
9 Without alternative, as time failed to pay off house loan? (8)
AMORTISE : OR(a conjunctive word introducing an alternative) contained in(Without …, …) anagram of(… failed) AS TIME.
10 Soldier returning at his time of day to light up (6)
IGNITE : Reversal of(… returning) GI(an American soldier) plus(at) NITE(a GI’s;his American spelling of “night”;time of day).
11 Deliver beastly baby Brown (4)
FAWN : Double defn: 1st: To give birth to a baby deer; and 2nd: …, the colour.
12 Capital is accessible in change, maybe (10)
COPENHAGEN : OPEN(is accessible) contained in(in) anagram of(…, maybe) CHANGE.
13 Kept lover‘s ring, see, after performance (6)
GIGOLO : [ O(something shaped like a ring) + LO(see, as in “lo and behold!”) ] placed after(after) GIG(a one-time performance, usually by musicians or comedians at a club or concert).
14 Chemical weapon — face wind (5,3)
NERVE GAS : NERVE(face;audacity) + WIND(gas passed by one with too much of it in his/her stomach).
16 Garden flowers: the latest picture (8)
SNAPSHOT : SNAPS(short form for “snapdragons”;garden flowers) + HOT(the latest in trends).
19 You, having changed just one letter, should get a medal! (6)
SILVER : “solver”(one trying this puzzle, refered to by the setter by “you”;the second person pronoun) with “o” replaced by “i”(having changed just one letter).
Defn: … for second position in a competition.
21 Hobo’s besotted with bling and spirits (10)
HOBGOBLINS : Anagram of(… besotted …) [HOBO’S plus(with) BLING].
23 Lift Democrat offensive (4)
DOFF : D(abbrev. for a member of the Democratic Party) + OFF( “rotten” or “hostile”;offensive).
Defn: … one’s hat in salutation.
24 Investigate former area of England (6)
SUSSEX : SUSS(to investigate;to figure out) + EX-(prefix indicating “former”;once before).
25 Deterrent to one who’d steal a broadsheet setter, possibly not Paisley? (5,3)
GUARD DOG : [ “Guardian”(the broadsheet newspaper) + DOG(an example of which;possibly, is the setter) ] minus(not) “Ian”(Rev. Paisley, Northern Irish leader).
26 A tailor at times rues working in topless coat (8)
MEASURER : Anagram of(… working) RUES contained in(in) “smear”(a coat of substance spread over a surface, eg. glazing on pottery) minus its 1st letter(topless …, in a down clue).
Defn: … has to do as this.
27 Political supporter drops resistance to a new Far Eastern corporation (6)
TOYOTA : “Tory”(a supporter of the Conservative Party) minus(drops) “r”(abbrev. for “electrical resistance” in physics) + anagram of(… new) TO A.
Defn: …, Japanese to be exact.
Down
2 “Our Mutual Friend”, say, which makes the set? (9,6)
COMPANION VOLUME : Cryptic defn: The novel by Charles Dickens cryptically refered to;say, as a book;volume about a companion;friend.
Defn: That which would complete a set of, say, books. (And this book, the last to be completed by Dickens can be said to complete the set of books authored by him.)
3 Battle ruin given energy, new energy (7)
MARENGO : MAR(to ruin;to damage) plus(given) E(abbrev. for “energy” in physics) + N(abbrev. for “new”) + GO(energy;drive).
Defn: … between Napoleon’s and the Austrian armies.
4 This local woven material (9)
SAILCLOTH : Anagram of(… woven) THIS LOCAL.
5 Belittle mass held in NEC complex (7)
CHEAPEN : HEAP(a pile;a mass of stuff) contained in(held in) anagram of(… complex) NEC.
6 Old poet changing start of The Hunter (5)
ARION : Replace(changing) the 1st letter of(start of) “Orion”(the mythological hunter) with “a”.
Defn: …, in Ancient Greece.
7 Type of make-up, one in fancy pack with lace at the end (7)
PANCAKE : AN(in grammar, an article for “one” of something) contained in(in) anagram of(fancy) PACK plus(with) the last letter of(… at the end) “lace “.
Defn: A cosmetic or theatrical make-up made of powder compressed into a pancake shape. The term was actually a trademark name.
8 For Annette, worth remaking a picture (2,3,10)
ON THE WATERFRONT : Anagram of(… remaking) FOR ANNETTE, WORTH.

15 Proof of saint’s transfiguration in balance (9)
RESISTANT : Anagram of(…’s transfiguration) SAINT contained in(in) REST(the balance;that which is left over).
Defn: Being able to withstand;being impenetrable, impervious or invulnerable, as is “foolproof”.
17 Brad, while on Universal set, ignoring film star group (7)
PEGASUS : PEG(not Pitt, but a small nail that can be used as a peg) + AS(while;during that time, as in “as you go through life”) placed above(on, in a down clue) U(abbrev. for “Universal”) + “set” minus(ignoring) ET(the Steven Spielberg sci-fi film).
Defn: …; a constellation.
18 Tense construction worker set off (7)
TRIGGER : T(abbrev. for “tense” in grammar) + RIGGER(a construction worker skilled in the use of pulleys, lifting gear, cranes, etc.)
20 Musical Holiday (25 March) (4,3)
LADY DAY : Double defn: 1st: Nickname of Billie Holiday, jazz singer; and 2nd: The day of the feast of the Annunciation.

22 Contender who puts Jack in his place? (5)
BOXER : Cryptic defn: What you might call the person who puts the clown in a Jack-in-the-box toy into his, well, box.
Thanks Brummie & scchua [wot no quiz?]
Must have been on the right wavelength as most of this went in without too much trouble. Of course, Our Mutual Friend has the right number of letters to be an anagram, which held me up for a while. I’m not sure why A?I?N had to be some (obscure?) old poet when there are lots of other possibilities.
ON THE WATERFRONT a good anagram and a cracking film!
Thanks Brummie and scchua
scchua, what has happened to the picture puzzles? I love them.
8d only needed a y for it to be the name of a girl with whom I went to uni ( I nearly typed “that I went to uni with”).
I liked this and managed to complete it slowly, though missed some of the subtlety in answers such as COMPANION VOLUME and LADY DAY. Hadn’t heard of ARION before, I think my only new word. Favourites were PANCAKE, CHAMPION and SUSSEX.
Thanks to scchua and Brummie.
Thanks Brummie and scc hua. Enjoyable solve waiting for flight to Naples.
Favourite clue for 12ac: capital found south of Wellington.
[Hi, Robi and Kevin, sorry, strapped for time at the moment, between trips, catching up on backlog and planning for the next trip. Will resume asap, as they say.]
Slow but got there in the end. Took a long time to get HOBGOBLINS – silly me. Plenty of excellent clues including NERVE GAS, FAWN, SUSSEX and GIGOLO. Many thanks to Brummie and scchua.
@Marienkaefer4, your new definition sounds intriguing, but could you parse it please?
RogerBear : Copenhagen was the Duke of Wellington’s horse – a brilliant clue.
Enjoyed this – tough in parts. Never heard of SNAPS as short for snapdragons, though the answer was obvious.
Thanks to Brummie and scchua
I meant to say clue.
scchua
All is forgiven
Look forward to next one
A lot of horse play in here? Copenhagen, Marengo, Silver, Trigger, Pegasus and Champion
…..and Boxer
Tyro @11, don’t forget Gigolo.
and Arion.
Tyro@11: Well spotted! I hadn’t really enjoyed this crossword, but I like it more now.
Thanks to Brummie and scchua.
Marengo was Napoleon’s horse,which makes for two great generals and two TV cowboys, but who were or are Champion and Boxer?
Thanks Brummie and scchua, and well spotted Tyro, even if the horse theme was coincidental. My way in was also by COPENHAGEN. Many good clues, I liked RESISTANT in particular.
Valentine @16, Champion-the-Wonder-Horse.
Champion the Wonder Horse was the on-screen companion of singing cowboy Gene Autry …….and Boxer was the draft horse in Animal Farm
and here is Boxer the horse of Animal Farm.
Cookie@18 pipped me to the post by a “short head or nose”
Valentine@16,
And Boxer in Animal Farm.
Tyro @21, such fun, amazing that Brummie could fit them all in. It makes up for the lack of scchua’s quiz.
Tricky, educational but fair, so just the kind of crossword I like. I’d never heard of ARION, which was last in because I had to check it. For once I spotted the theme, and most of the themed solutions. Liked GUARD DOG. COPENHAGEN and MARENGO will be familiar to Radio 4 listeners as the “Warhorses of Letters”
Thanks to Brummie and scchua
Thanks to Brummie and scchua. As usual I missed the theme but otherwise proceeded fairly quickly. I needed help parsing CHAMPION (though I have read Margery Allingham) and parsed RESISTANT without catching the “proof” connection. Great fun.
Safely in Naples and pleased to see I inadvertently stumbled across the theme. Another version of the clue is of course capital found north of Copenhagen…
Probably just chance, but Hobgoblins Equine Reproduction Centre seems to be well known.
OMG – a theme I missed! No change there, then.
Blimey, I spotted the theme–again! I found this hard but I thought it an excellent puzzle. I loved COMPANION VOLUME and LADY DAY but there was a lot more to enjoy than these two.
Thanks Brummie.
P.S. Wasn’t BOXER one of Dan Archer’s horses as well?
Thanks Brummie and scchua
Came to this late (and tired) and really didn’t enjoy it. I won’t bore everyone by listing the clues I didn’t like. I missed all the horses, which would have redeemed it for me if I had seen the theme.
I did like very much MARENGO and GUARD DOG.
Never met SNAPS as an abbreviation before (and I’m a gardener who grows them).
Missed the theme again,keeping my record nearly perfect!
Re 10: I am an American, and I spell the word “night.” If I ran across the word “nite” in a sentence I would not know what it meant.
scchua, I was lucky enough to get 8D (ON THE WATERFRONT) first, 12D (COPENHAGEN) being my second. The only theme words I saw were CHAMPION, TRIGGER and PEGASUS. I give myself credit for seeing even these because I never spot themes! (I read Animal Farm many years ago but forgot the name of the horse.) More credit goes to Brummie for incorporating all these interesting horses so smoothly into the grid.
I enjoyed this crossword, which I found not too easy and not too difficult. (Sorry it wasn’t your cup of tea, muffin.) The only clue that startled me a bit was 10A (IGNITE), and I feel vindicated by slipstream @32, who also queried ‘nite’ (which should be ‘night’ unless the informal / illiterate form is indicated).
Thanks to Brummie for this well-constructed themed puzzle, and to scchua for the blog.
I meant 12A, not 12D, for COPENHAGEN in my post @33.
Alan Browne @34
To be fair to Brummie at 10A he does indicate that “nite” is “at his time of day”, “he” being a GI.
The SOED has
nite n?it ? noun. colloq. (chiefly N. Amer.). e20.
Night.
I thought this was OK.
Brendan @35
Yes – I totally understood ‘his’ to refer to the GI but, amazingly, nothing like ‘N. Amer.’ is indicated in Chambers. Collins doesn’t even recognise the word ‘nite’. If I’d had the SOED I would not have made my comment, but having read slipstream’s comment I felt confident enough in doing so.
Thanks for the hard info you had to hand. Yes, it’s ok.
Thanks Brummie and scchua
Only got to check this off today after actually doing it on the day. I also had a slow kickoff into it, with IGNITE (obviously had no issues with the US reference to NITE) closely followed by ON THE WATERFRONT which then opened up the right hand side of the puzzle.
Entered LADY DAY from the crossers, but had to look up the significance of March 25 and had to come here to find the Billie Holiday link.
Did go looking for the theme, as Brummie often does have one – afraid that it was too clever for me, only would have known half of them at most anyway.
Finished down in the SW corner with SUSSEX (‘former county’ threw me for a while, forgetting that it had been split), BOXER (clever after finally seeing what was going on) and MEASURER as the last one in.
Alan @36
I saw your comment on tomorrows crossword that you’d replied 😉
My SOED is the electronic version as an “Add on” to WordWeb Pro. It’s very good but if I remember works out at about £55. (Well worth it though)
slipstream @32, as a fellow American (Boston area) I agree, “nite” is not common usage here, though it does occasionally crop up in advertising and product names.
I found this to be on the tough side, mostly due to my own ignorance and slow-wittedness. On the Waterfront was FOI, after which the right side went in slowly but steadily. The left side was much more of a struggle for some reason. And, as usual, I completely missed the theme.
I think 11A could almost be called a triple definition (Deliver | beastly baby | Brown), since “fawn” refers both to the act of giving birth and the offspring itself.
Overall, I found this to be a clever, well-crafted and enjoyable puzzle (in spite of my difficulties). Thanks to Brummie and to scchua.
Thanks scchua and Brummie.
I made heavy going of this – my way in was NERVE GAS – and took an age to finish.
ARION was new to me but solved that relatively early due to the simple clue.
I had completely forgotten the LADY DAY nickname for Billie so thanks for explaining that.
Although I only pencilled it in, I considered CHASTEN for a long time at 5dn on the basis that “mass held” could be HAS T(onne). The Tonne is a unit of mass and the word “in” is already there as an inclusion indicator.
So that meant that COPENHAGEN was one of my last in and I failed to spot the theme completely.
Plu ca change……..