Thanks to Matilda for this enjoyable Quiptic. Definitions are underlined in the clues.
Across
1 Arms, bottom and tops of neck and legs (7)
ARSENAL : ARSE(bottom/a person’s rear end) plus(and) 1st letters, respectively, of(tops of) “neck and legs“.
Defn: A collection of …
5 Genuine training for beginner (7)
INGENUE : Anagram of(… training) GENUINE.
Defn: … in the ways of the world, namely, one who is innocent and wholesome. But obviously, no-one’s one forever.
The word originates from France, as does this ingénue:

9 What goes round and round and round (5)
ROTOR : A palindrome(goes … round)
10 Leaders of Montenegro and Norway alongside EU over working plan (9)
MANOEUVRE : 1st letters, respectively, of(Leaders of) “Montenegro and Norway” plus(alongside) anagram of(… working) EU OVER.
11 Tidy and mostly set right mess (10)
STRAIGHTEN : Anagram of(… mess) [ “and” minus its last letter(mostly) + SET RIGHT].
12 Sell whip (4)
FLOG : Double defn.
14 Oops, Tories so affected by bodily weakness! (12)
OSTEOPOROSIS : Anagram of(… affected) OOPS, TORIES SO.
Defn: … in the form of brittle bones.
18 Gender-bending female making a mistake and moving on (12)
TRANSFERRING : TRANS(short for transgender/gender-bending) + F(abbrev. for “female”) + ERRING(making a mistake).
Defn: …, say, from one football team to another.
21 Head off, being fussy and unpleasant (4)
ICKY : 1st letter deleted from(Head off) “picky”(being fussy/nit-picking).
22 Be ruthless arranging street furniture (3,7)
BUS SHELTER : Anagram of(… arranging) BE RUTHLESS.
Street furniture cum street art.

25 Weaken the heart of wounded stoat (9)
UNDERMINE : Central 3 letters of(the heart of) “wounded” + ERMINE(a stoat, the animal valued for its fur).
26 Frothy beer left to rise (5)
REBEL : Anagram of(Frothy) BEER + L(abbrev. for “left”).
27 Old writing implements exceed top price (7)
EXPENSE : EX-(prefix signifying “old”/former) + PENS(writing implements) + 1st letter of(… top) “exceed“.
28 Refurbishing necessary? Not as background (7)
SCENERY : Anagram of(Refurbishing) [ “necessary” minus(Not) “as“].
Down
1 An angry sort of clue (6)
ACROSS : A(one in number/an) + CROSS(angry).
2 Humour posed with fury (6)
SATIRE : SAT(posed, as an art or photographic model does) plus(with) IRE(fury/wrath).
Defn: A type of …
3 Achieving 50% reduction of narcotic issues in smug self-admiration (10)
NARCISSISM : Letters in the 2nd halves, respectively, deleted from(Achieving 50% reduction of) “narcotic issues in smug“.
… so called after this lad:

4 Fluid laboratory drained with speed (5)
LYMPH : “laboratory” minus all its inner letters (drained) plus(with) MPH(abbrev. for “miles per hour”, a measure of speed).
Defn: Colourless fluid containing white blood cells.
5 Look up, interrupting central host (9)
INNKEEPER : Reversal of(… up, in a down clue) PEEK(to look quickly or furtively) contained in(interrupting) INNER(central/at the core).

6 Delight in good shelter (4)
GLEE : G(a “good”) + LEE(shelter from wind or weather provided by an object).
7 Original first writer (8)
NOVELIST : NOVEL(original/never before) + IST(1st, with the Roman numeral substitution).
8 Green salad is primarily eaten to invigorate (8)
ENERGISE : Anagram of(… salad) GREEN + IS + 1st letter of(primarily) “eaten“.
13 Concoct or generate source of citrus (6,4)
ORANGE TREE : Anagram of(Concoct) OR GENERATE.
Defn: …, specifically, oranges.
15 Person responsible put to death about four (9)
EXECUTIVE : EXECUTE(to put to death) containing(about) IV(Roman numeral for “four”).
16 Bearing gift, having time for papa (8)
ATTITUDE : “aptitude”(a gift/a natural talent for doing something) with T(abbrev. for “time”) replacing(having … for) P(letter signified by “papa” in the phonetic alphabet).
17 British Rail left bankrupted over drunk (6,2)
TANKED UP : Anagram of(… over) [“BR”(abbrev. for “British Rail”) deleted from(left) “bankrupted” ]
19 Where Arabs may be steady (6)
STABLE : Double Defn: 1st: Place where you may find Arabs, a breed of horses originating from, well, Arabia.
20 Get well following nil by mouth (6)
ORALLY : RALLY(to get well/to be on the road to recovery) placed below(following, in a down clue) O(letter denoting 0/nil).
23 Small mushrooms served up in glasses (5)
SPECS : Reversal of(… served up, in a down clue) [ S(abbrev. for “small”) + CEPS(edible European mushrooms) ].
Defn: … worn to make you see better, in its shortened form.
24 Part of the environmental press (4)
IRON : Hidden in(Part of the) “environmental“.
Defn: …, say, clothes.
God lord, am I the first to comment? Wow, an unexpected pleasure after the expected pleasure of a Matilda crossword.
Arsenal! Amongst a host of deliciously clever clueing (REBEL, TANKED UP, SPECS, NARCISSISM, EXECUTIVE to name but some) ARSENAL was the champ!
Thanks to Scchua for the blog – and heartless gratitude to Matilda, for making me chuckle on a Monday morning when there’s so much grim and depressing news around…
Hearty gratitude, that should be! Blasted predictive toxt….
Thanks Matilda and scchua
I found this quite a lot more difficult than a typical Quiptic – the SW in particular was blank for some time.
Favourites were INGENUE, UNDERMINE and ORALLY.
I didn’t like the “in” in the 6d clue.
HeartFELT!!! Ok, I’ll get my coat…..
It was harder than today’s Cryptic, but I enjoyed it very much.
Whether this crossword was on the harder side of the Quiptic spectrum, I’m not sure. To each their own.
I didn’t find the puzzle particularly difficult, but I did find it really enjoyable with a host of top clues.
The only thing I wasn’t sure of was the anagram indicator in 17dn (‘over’) but I couldn’t be bothered too much.
Overall, I found the clueing very consistent (as ever with Matilda) – wish it were an Everyman!
muffin, I know you often put a question mark to the word ‘in’ as it is used here (in 6d).
I’m not fully sure why because it is quite a normal link word. Have a look at this article on Alberich’s website: http://www.alberichcrosswords.com/pages/linkwds.html Somewhere halfway he says something about the use of ‘in’. Coincidentally, it looks like Matilda does it right today.
Many thanks to scchua (for the blog) & to Matilda (for today’s quality crossword).
Thanks to Matilda, and to Scchua for the parsing of 9a and 16d, my LOI (I kept seeing tribute = gift, even though I knew it couldn’t work).
I couldn’t get the link provided by Sil@6 to work, but I agree with Muffin@3 regarding the use of “in”. In the case of today’s 6d, an abbreviation for “good” and a word meaning “shelter” are found in a word meaning “delight.” However, I’m aware that “in” is often just a linking word, so I won’t complain too much. A good puzzle overall.
To those, like DaveinNCarolina, who cannot get into the link I gave @6:
After clicking on it, the name appearing at the top of your screen (in my case, in Google Chrome) has a few extra characters at the end that shouldn’t be there.
Just delete what comes after .html [which is apparently %C2%A0, whatever that means] and then it’ll be fine.
Sil @8
I have edited the link in your comment @6 and it now works correctly.
I also got stuck in the SW for a while, and I didn’t manage to parse 17d, so I’d agree that it seemed a bit harder than past Quiptics, but not unreasonably so.
I wouldn’t have described a bus shelter as furniture — I think of furniture as what the French call “meuble” (movable). But the wordplay was clear, and a bit of latitude of this sort lets the setter create more enjoyable clues.
Ted @10
All sorts of street items – pillar boxes, telephone boxes, seats etc. etc. – are, for some reason I don’t know, known as “street furniture”.
Gaufrid @9
The link still doesn’t work for me.
muffin @11 — Thanks for the information. It’s not a phrase I’d heard before. Another item on the long list of linguistic differences between US English and the English of the Commonwealth, I guess.
muffin @11
I have just tried it again and the link works using Chrome. Perhaps you still have the original erroneous link in your browser’s cache.
OK, the link works now and leads to an interesting article, so thanks, Sil. My mind still processes “in” as [wordplay] is found in [definition], but I accept that that’s just me.
Thanks to Matilda and sschua.
That was good; I’ll watch out for Matilda again. The “wounded stoat” charmed me for some reason, and there were others, both devices and surfaces, to admire.
I think that it’s not just you. Your way of thinking is fully justifiable too. So, in my opinion, it is still a what they call a ‘non-directional’ link word.
But the main thing is, in my opinion, that it is an accepted link word.
And mind you, both Alberich/Klingsor and Monk are seen as pretty Ximenean setters.
That said, if muffin doesn’t like it, there is probably a reason for it and who am I to start an argument?
[thank you Gaufrid, for making the link work]
This was an enjoyable Quiptic.
LOI was TANKED UP – it took a while fro the penny to drop.
Thank you B+S
I found this quite hard, and absolutely loved it. Full of that crucial grrrr/pleasure when you twig, and something previously obtuse is suddenly easy, and why didn’t you notice it an hour ago … or yesterday?
Thanks to setter and blogger both.