An enjoyable crossword with clues that have meaningful surfaces. Thank you Eccles. Definitions are underlined in the clues.
Across
1 Roles reversed in tie (5)
STRAP : Reversal of(… reversed) PARTS(roles, say, in a play or movie).
4 Demote fraudulent dog warden (9)
DOWNGRADE : Anagram of(fraudulent) DOG WARDEN.
9 Prevents Russian flower arrangements having flower missing (9)
OBSTRUCTS : OB(a river/flow-er in Russia) + “structures”(arrangements of the parts or elements of something complex) minus(having … missing) “Ure”(another flow-er, this time in Yorkshire, England).
10 Newscast from comedian Frank: he’s possibly going downhill quickly (5)
SKIER : “n,n”(abbrev. for “new” x 2) deleted from(cast from) “Skinner”(Frank, English comedian).
11 She’s changed the locks of broken box and reopened lid (8,6)
PEROXIDE BLONDE : Anagram of(broken) [BOX plus(and) REOPENED LID] .
13 This set upset people who believe in God (7)
THEISTS : Anagram of(… upset) THIS SET.
15 Northern entertainers encountering nothing but resistance for northern style (3,4)
ART DECO : [ “Ant” & “DEC” ](TV entertainers from Newcastle-upon-Tyne, North England) plus(encountering) O(letter representing 0/nothing) with “r”(abbrev. for electrical resistance in physics) replacing(but … for) “n”(abbrev. for “northern”).
Defn: …, of visual arts, architecture and design.

17 Huge area of mountains left deserted between Spain and the borders of India (7)
EURASIA : “Urals”(mountain range running through Russia and Kazakhstan – and part of the conventional boundary between the European and Asian continents) minus(… deserted) “l”(abbrev. for “left”) contained in(between) [ E(International Vehicle Registration Code for Spain) plus(and) 1st and last letters of(the borders of) “India” ].
Defn: … consisting of the combined landmass of the European and Asian continents.
19 Unionists accepting substantial Tory monetary offer, initially, but this should clear things up (4,3)
DUST MOP : DUP(abbrev. for the “Democratic Unionist Party”, a unionist political party in N. Ireland) containing(accepting) 1st letters, respectively, of(…, initially) “substantial Tory monetary offer “.
The clue makes reference to the Tory-DUP deal that enabled a minority government in the UK?
21 What hardware store is doing for Christmas treat (8,6)
STOCKING FILLER : STOCKING(what a store is doing/amassing/increasing its supplies for sale especially before an occasion such as Christmas) + FILLER(a substance for filling in cracks in a surface, especially before painting it, something a hardware store would stock and sell).
Defn: … in the form of a small present suitable for putting in a Christmas stocking.
And yes, Christmas is due. A Merry Christmas to all. And A Happy New Year!
25 Evacuation from east? So be it: to the west (5)
ENEMA : E(abbrev. for “east”) + reversal of(…: to the west, from the east, in an across clue) AMEN(word meaning “so be it”, uttered at the end of a prayer or hymn).
Defn: A procedure to achieve evacuation of the bowel by the injection of fluid into the rectum.
26 3 runs behind schedule, then 4. Declared as a result (9)
THEREFORE : “three”(3) with “r”(abbrev. for “runs” in cricket scores) moved to a later position(behind schedule) plus(then) homophone of(…. Declared) “four”(4).
27 Strike Scot, perhaps, from behind, extremely raucously, giving jingoistic shout (6,3)
BATTLE CRY : BAT(to strike with, well, a bat) +reversal of(…, from behind) CELT(an example/perhaps of this ethnolinguistic group is a Scot) + 1st and last letters of(extremely) “raucously“.
28 Besmirch brand admitting scam, finally (5)
SMEAR : SEAR(to mark with a hot branding iron) containing(admitting) last letter of(…, finally) “scam“.
Down
1 Broken golf club – one engaged in another sport (4,6)
SHOT PUTTER : SHOT(broken/ruined, as in “this engine is shot”) + PUTTER(a golf club that’s used near or on a putting green).
Defn: …, more specifically a sports/athletics field event.
2 Book demonstrates what to do if a fault occurs (7)
RESERVE : Double defn: 1st: As in “to book a restaurant table”; and 2nd: Re-serve/what a tennis, badminton, table-tennis player has to do if his/her first service is at fault/not in accordance with the rules of the game.
3 Fits answer into text message sent on behalf of someone else (9)
PAROXYSMS : A(abbrev. for “answer”) contained in(into) [ PROXY SMS](sms/abbrev. for “short message service/a text message from one mobile phone to another sent by proxy/on behalf of someone else).
4 Cut up and passed around cocaine (5)
DICED : DIED(euphemistically termed “passed”) containing(around) C(abbrev. for “cocaine”).
5 ‘Instrument used to be demanding’, claims British orchestra leader (9)
WASHBOARD : WAS(used to be) + HARD(demanding/difficult) containing(claims) [ B(abbrev. for “British”) + 1st letter of(… leader) “orchestra“].

6 Terrible gout curbing son’s enthusiasm (5)
GUSTO : Anagram of(Terrible) GOUT containing(curbing) S(abbrev. for “son”).
7 King of Crete’s daughter and I are drunk (7)
ARIADNE : Anagram of(… drunk) AND I ARE.
8 King’s Head is going downhill, serving nobleman (4)
EARL : LEAR(King in Shakespeare’s play of the same name) with its 1st letter(…’s Head) moved to a later position(is going downhill, in a down clue).
And the King’s Head could very well be a pub serving nobility.
12 Bookseller quoted composer of musicals (10)
COLPORTEUR : Homophone of(quoted) Cole Porter(American composer of musicals).
14 Starts to suspect thief is hiding in loft, in fact (9)
STATISTIC : 1st letters, respectively, of(Starts to) “suspect thief” + [IS contained in(hiding in) ATTIC(loft/a room directly under the roof of a house).
Defn: … or piece of data.
16 Irritability shown during contest in Essex (9)
TESTINESS : Hidden in(shown during) “contest in Essex“.
18 Minister again provides refuge (7)
RETREAT : RE-TREAT(to treat/to minister to again).
20 In some form, most ale contains this (7)
MALTOSE : Anagram of(In some form) MOST ALE.
Defn: A sugar formed from malted grain such as barley – in brewing ale, yeast converts maltose into alcohol and carbon dioxide, so that ale contains little or no maltose
22 Stroke stomach and top of leg (5)
CRAWL : CRAW(the stomach of an animal) plus(and) 1st letter of(top of, in a down clue) “leg“.
Defn: A stroke/a series of a sort of movement of arms and legs used in swimming.
23 Nerdy Yorkshireman, say, with no time for reflection (5)
GEEKY : Reversal of(… for reflection) [“tyke”(a person from Yorkshire) + EG(abbrev. for “exampli gratia”/for example/say) minus(with no) “t”(abbrev. for “time”)].
24 Maybe protest over bare-knuckle boxing (4)
VERB : Hidden in(… boxing/enclosing) “over bare-knuckle“.
Defn: In grammar, a part of speech, an example of which /maybe is “protest”.
So many excellent clues – where to start? One of my favourite crosswords of the year.
SKIER – great use of NEWSCAST
PEROXIDE BLONDE – lovely surface with nothing related to final answer.
EURASIA – another great and clever surface.
RESERVE & CRAWL I also liked a lot. Then there was STOCKING FILLER …
COLPORTEUR was new to me and found using a word fit. What a great word. Amazed it existed.
Thanks to Eccles and scchua and a happy Christmas to all.
Great stuff, Andy. This was great fun from start to finish with accurate cluing and lovely surfaces. I needed Scchua’s help to parse 9a and 12d was a new word for me.
The pedant in me thinks that 2d should really say “… what to do sometimes if a fault occurs”.
As Hovis says, there were many excellent clues. I’ll plump for 11a as my favourite.
Many thanks to Eccles and to Scchua.
Enjoyed this one in retrospect far more than I did at the time of solving when it all seemed somewhat disjointed.
I didn’t know the unfortunate name given to Yorkshiremen, nor the bookseller, but both were easily arrived at from the wordplay.
Think I’ll go along with RD and give top honours to 11a.
Thanks to Andy and to scchua – a very happy Christmas to both of you.
COLPORTEUR a new word on me, but derived from the crossers and wordplay, otherwise all worked out from the neat clues without too much head scratching. Nice puzzle. The topical STOCKING FILLER my favourite. Thanks Eccles and Scchua
does COLPORTEUR really sound like Cole Porter?
Not in any way that I’d pronounce the 2 of them that’s for sure unless I was adopting a Clouseau-esque French accent for the latter!
That aside a very enjoyable puzzle indeed so many thanks to Eccles and to scchua for the blog.
Many thanks to scchua for the fine blog – I don’t think our paths have crossed before? – and to all who commented. Redddevil, it is a very fair question (although I am not normally bothered by the requirement for exact homophones, but for a less well-known word I wanted it to be quite close), but I listened to the pronunciation on the Collins website at https://www.collinsdictionary.com/dictionary/english/colporteur and to me it sounded very similar, but you can judge for yourself if you wish.
Merry Christmas to everyone, and a big thank you for continuing to do the Independent puzzles. I look at the quality of the other Indy setters’ puzzles, and am very proud to be part of the team. The number of people going on to the site – and hence the advertising revenue that can be obtained – is vital for the crossword to continue. Fifteensquared in also a really important resource, so thanks to Gaufrid and all those who help run it and blog.