Solution to 27A 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.
ACROSS
1 A despicable person consuming ecstasy and heroin in bar (8) ALEHOUSE {A}{L{E}{H}OUSE}
5 Universal drive to tackle onset of rampant surge (6) UPRUSH {U}{P{Ra...t}USH}
10 Royal shot in the arm, beheaded in bed (5) ROOST {R}{bOOST}
11 Scrap between advocate and defamer (9) BACKBITER {BACK{BIT}ER}
12 Following kiss, female removes thongs (4-5) FLIP-FLOPS {FLIP}{F}{LOPS} ( Correction - {F}{LIP}{F}{LOPS} - Ser comments)
15 Mean associate with bad temper is railing (7) PARAPET {PAR}{A}{PET}
18 Buzz on starting to drink liquor, commonplace (7) HUMDRUM {HUM}{Dr..k}{RUM}
22 Honest head of co-operative bank (5) CLEAN {Co...e}{LEAN}
24 It stabilises mood in stress or tension (9) SEROTONIN*
25 Knowledge of training routine keeps daughter fit essentially (9) ERUDITION {ROUTINE}* over {D}{fIt}
26 Chemical compound in Botox identified (5) OXIDE [T]
27 Extremely small worm in toboggan (6) S?E?G? (Addendum - SLEDGE {SmalL}{EDGE} - See comments)
28 Frozen layer in Italian sandwiches, cheese perhaps (3,5) ICE SHEET {I{CHEESE*}T}
DOWN
1 Insecure director involved in a fight (6) ADRIFT {A}{D}{RIFT}
2 Sexual excitement caused by isometric exercises (9) EROTICISM*
3 No longer in contention, having left management? (3,2,3,7) OUT OF THE RUNNING [DD]
4 Vehicle overturned, primarily spilling fuel in layer below surface (7) SUBSOIL {BUS<=}{Sp...g}{OIL}
6 Circulating pic or bulletin as promotion (6,9) PUBLIC RELATIONS* Semi&lit
7 Posh model beset by love before (5) UNTIL {U}{N{T}IL}
8 Man tried altering chromosome in genetic make-up (8) HEREDITY {HE}{TRIED*}{Y}
9 Misbehaves in Thailand, abandoning discretion on drink (4,2) ACTS UP {tACT}{SUP}
16 Open relationship keeps lover essentially unhappy (9) PLAINTIVE {PLAIN}{TI{loVer}E}
17 Passes across hotel in woods (8) THICKETS {T{H}ICKETS}
19 Weirdo in film, confining female on island (6) MISFIT {MIS{F}{I}T}
20 Insane spread of Omicron (7) MORONIC*
21 Sinister criminal is isolated in enclosure (6) INSERT SINisTER*
23 Weep as old guy drops dead (5) EXUDE {EX}{dUDE}
Reference List
Ecstacy = E, Heroin = H, Universal = U, Royal = R, Following = F, Female = F, Woman = W, Sex appeal = IT, Mean = PAR, Associate = A, Bad temper = PET, Caught = C, Daughter = D, Italian = IT, Director = D, Posh = U, Model = T, Chromosome = Y, Thailand = T, Hotel = H, Female = F, Old = EX, Dead = D