Solution to 12A 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 Say, green pass belonging to us (6) COLOUR {COL}{OUR}
5 Mark ate bananas, swallowing last bit of apple in pan (3,5) TEA MAKER {MARK+ATE}* over {a..lE}
9 Custom officer sent maid out (8) TIDESMAN* Should be 'Customs officer'
10 Say, Don Quixote lost diamonds in drilling tool (6) REAMER dREAMER
11 Unprofessional agency manager annoyingly tactless, ended up raking issues slandering HR leaders (10) AMATEURISH Acrostic
12 Egad! Twenty prime positions for exit (4) G?T? (Addendum - GATE {eGAd+TwEnty} - See comments)
13 Revulsion is stated in a funny manner (8) DISTASTE*
16 Mathew, "Is dominance camouflaging sapience?" (6) WISDOM [T]
17 Preservation of 'High Citadel' (6) UPKEEP {UP}{KEEP}
19 Influenced a cute date to go berserk after ditching European (8) ACTUATED {A+CUTE+DATe}*
21 Some delicate matter turned up referring to itself? (4) META [T<=]
22 Cunning tweets rise, running wild (10) STREETWISE*
25 The Curies' found it right before good man quit arena (6) RADIUM {R}{stADIUM}
26 Show of hands or change of hands in assembly? (8) ELECTION E(-r+l)LECTION
27 Non-vegetarian food picked up with rum, gin and dessert essentially in convocations (8) MEETINGS {(~meat)MEET}{GIN*}{desSert}
28 What a hit indeed featuring island made famous by R.L. Stevenson (6) TAHITI [T]
DOWN
2 A powerful drug obliterating pain, in use maybe from the beginning (5) OPIUM Acrostic
3 Frank's buried in poverty (5) OVERT [T]
4 Sour words on the street? (7) RUMOURS {SOUR}* [RA]
5 Steel in building can be drawn out (7) TENSILE*
6 Telecast performance of pilots? (3,4) AIR SHOW {AIR}{SHOW} (Correction [DD] - See comments)
7 Distressed angel overwhelmed by continent's inability to feel pain (9) ANALGESIA {A{ANGEL*}SIA}
8 Pick up right song for conductor (9) ELECTRODE {ELECT}{R}{ODE} Pick up or just Pick?
14 Damn! Rascal nearly created mayhem (9) IMPRECATE {IMP}{CREATEd*}
15 Terrible threat is hovering over chief psychiatrist and mind doctor (9) THERAPIST {THREAT+IS}* over {Ps...t}
18 According to the Reverend, almost all criticise the Man of Letters! (7) POSTMAN (~most pan to postman)
19 By the sound of it, not having anyone to leave your belongings to is stifling (7) AIRLESS (~heirless)
20 Model bets about stealing extremely handsome Turner's number! (3,4) THE BEST {T}{BETS*} over {Ha...mE}
23 Say, Hermione Granger's humour - cold and hard-hitting at first (5) WITCH {WIT}{C}{Ha...g}
24 Strong beer (5) STOUT [DD}
Reference List
Pass = COL, Diamonds = D, European = E, Right = R, Good man = SAINT = ST, Hands = R,L, Model = T, Cold = C
