Across:
1 They’re checked (PLAIDS) in place (PL-) with first-class (-AI-) detective (-DS) (PLAIDS),
4 Murders (serial killers = BUTCHERS) Schubert (Schubert) composition (= anagram indicator) (Schubert = BUTCHERS),
9 Agrees to (ADOPTS) dig up (= reverse indicator) bone (-T- bone) and shell (pod = -DOP-) in South Africa (SA = A-S) (ADOPTS),
10 Try in the formal sense (ESSAY-) to get visa at The Centre (‘vISa’ at the centre = -IS-) open to travel (open to ‘Travel’ = -T) (ESSAYIST) writer (ESSAYIST),
12 3 down (‘dates’ = DATE S-) is central to 20 down? (central to ‘stamps’ = -TAMP-) (DATE STAMP) That’s one for the books, as it marks the day for the librarian! (DATE STAMP is used by librarian marking the day the book is due back),
13 Come to terms (AGREE) with month away from hatemonger (‘month’ away from ‘hatemonger’ = aeger = AGREE),
14 Is reluctant to hand over remote (remote CONTROL FREAK) to micromanager with a problem (CONTROL FREAK),
18 Sunblock (CURTAIN) for The Bellows (CALLS) (CURTAIN CALLS) – the types demanded by the people at play (CURTAIN CALLS),
21 Exercised one’s right (VOTED) reflecting (= reverse indicator) part of attitude to venture (part of ‘attituDE TO Venture’ = detov = VOTED),
22 One of the suits (EXECUTIVE) of four (-IV-) taken in by Lynch (lynch/murder = EXECUT-E) (EXECUTIVE),
24 Many (NUMEROUS) from university (-U-) bringing up (= reverse indicator) dreamy thoughts (rapid eye movement/dreamy part of sleep = REM = -MER-) overcome by common sense (nous = N-OUS) (NUMEROUS),
25 Smear (BEDAUB) queen (BED-) going to Australia (-AU-) with a follower (‘a’ follower is -B) (BEDAUB),
26 It won’t have escaped one following game (EASY PREY), as it’s not difficult (EASY) to contact the man upstairs (pray) we heard (= homophone indicator) (‘pray’ = ‘PREY) (EASY PREY),
27 Stick to (ADHERE) cheat dismissed by head teacher (‘cheat’ dismissed by ‘head teacher’ = header = ADHERE).
Down:
1 Take a risk on (PLAY DICE) Sive (John B Keane’s Sive = PLAY), Diana (DI-) and Clare (-CE) (PLAY DICE),
2 Breaking down (ABORTING), reversing (= reverse indicator) it (it = -TI-) in Bangor (Bangor) resort (= anagram indicator) (Bangor = ABOR-NG) (ABORTING),
3 Part of Somerset advertisement (part of ‘somerSET ADvertisement = setad) for return (= reverse indicator) (setad = DATES) of fruit (DATES).
5 A foreign (‘a’ foreign/French = UN-) criminal (= anagram indicator) uses Viper (uses Viper = SUPERVISE) to open door (open ‘Door’ = -D) (UNSUPERVISED) that’s not monitored (UNSUPERVISED),
6 Clean (CHAR-) a (-A-) bar (-BAN-) for 100 (Roman 100 = -C) (CHARBANC) from early bus (CHARABANC),
7 Political exile (ÉMIGRÉ) offering regime (regime) change (= anagram indicator) (regime = ÉMIGRÉ),
8 Would people lie about this (SETTEE) fix (SET-) picked up by swinger? (-TEE picked up by swinger/golfer) (SETTEE),
11 What The Boss does (CALLS THE TUNE) with requests (CALLS) for Born in the USA, for example (THE TUNE) (CALLS THE TUNE),
15 This will bring The Tourists back to where it all started (ROUND TRIP) and it’s in The Ticket perhaps? (ROUND TRIP ticket),
16 Fundamental (ULTIMATE) sect (cult) loses leader (c) (‘cult’ without ‘c’ = ULT-) and one (-I-) confidante (-MATE) (ULTIMATE),
17 Labs seem (labs seem) unusual (= anagram indicator) (labs seem = ASSEMBLE) to construct (ASSEMBLE),
19 A (A-) scene (scene/place = -VENUE) (AVENUE) out of The Road (AVENUE),
20 Frank’s (franks/STAMPS) auntie is out of Asti Spumante (‘auntie’ is out of ‘Asti Spumante’ = stspma = STAMPS),
23 First Picasso (first ‘Picasso” = -P-) in nude (nude) form? (= anagram indicator) (nude = U-END) (UPEND) Flip! (UPEND).