Solution to 21A has been deliberately left unsolved and is to be answered only by a non-regular / novice commenter, with proper annotation. Those who have answered earlier in the week, please give others a chance.
Open for anyone to answer, if not solved by 1 PM.
ACROSS
1 Ordeal after girl in bash turned most intoxicated (8) TIGHTEST {TEST}<=>{H{G}IT<=}
5 Powder in back of closet - salt containing cocaine (6) TALCUM {c...eT}{AL{C}UM}
10 & 13 What DJs want everyone to do? Comply with what they want (5,2,5,4) DANCE TO THEIR TUNE [C&DD]
11 Small sample of antivenom in a lab (7) NOMINAL [T]
12 Son ignored insult in match (5) LIGHT sLIGHT
13 See 10
14 Cash, say, spent in altering film (6,6) TENNIS PLAYER {SPENT+IN}*{LAYER}
21 Musical troupe is smooth, entertaining group with bit of blues (5,4) S?I?G ?A?D (Addendum - SWING BAND {S{WING}{Bl..s}AND} - See comments)
23 Abandon movement to secure independence (5) WAIVE {WA{I}VE}
24 A rebellious chap consuming ecstasy on island getting cold and pale (7) ANAEMIC {A}{M{E}AN<=}{I}{C}
25 Talk foolishly with woman after starting to become drunk (7) BLITHER {HER}<=>{Be...e}{LIT}
26 Crushed oilseed yielding ordinary oil (6) DIESEL oILSEED
27 Assess racehorse gambolling round shed (8) RESEARCH RACEHoRSE*
DOWN
1 Slightly drunk in bar, lazily cuddling date (6) TIDDLY {T}{ID{D}LY}
2 Ornamental tree in good tattoo on head (6) GINKGO {G}{INK}{GO}
4 Reasonable likelihood of force catching person (8,6) SPORTING CHANCE*
7 Perhaps as bouncy as cylindrical channel markers (3,5) CAN BUOYS*
9 Bend a rule craftily to secure position as expected (14) UNDERSTANDABLE {BEND+A+RULE}* over {STAND}
15 Faultless liberal beginning to irritate the wily rogue (4-5) LILY-WHITE {L}{Ir...e}{THE+WILY}*
19 Nobody backs film star releasing flops essentially (6) CIPHER {PIC<=}{HERo}
20 Make out with hot model over area around fireplace (6) HEARTH {HEAR} with {H}{T}<=
22 Easy putt in golf that is over millimetres (5) GIMME {G}{I{MM}E}
Reference List
Girl = G, Cocaine = C, Son = S, Independence = I, Ecstacy = E, Island = I, Cold = C, Ordinary = O, Bar = T, Date = D, Good = G, Runs = R, American = A, Male = M, Joint = J, British = B, Liberal = L, Supporter = BRA, Hot = H, Model = T, Golf = G