Across:
1 Modern artistic movement (CUBISM) and key (music key -B-) in rock (= anagram indicator) music (music = CU-ISM) (CUBISM),
4 Olga (Olga = O-L GA-) protects one (-I-) mammoth (huge) initially lacking (h) (‘huge’ without ‘h’ = -UGE) (OIL GAUGE) instrument to calculate what’s happening in pressurised environment (OIL GAUGE),
9 Sued (sued = E-DUS) wrong (= anagram indicator) officer (executive officer = -XO-) involved (EXODUS) in mass emigration (EXODUS),
10 Incorrectly accounts for (MISREADS) side arms (side arms = MISREADS) going off (= anagram indicator),
12 Roman Catholics release carol (‘Roman Catholics’ release ‘carol’ = manthoics = MACINTOSH) for the winter season perhaps (MACINTOSH),
13 Sommelier dismisses soil (‘sommelier’ dismisses ‘soil’ = mmeer = EMMER) for the wheat that’s used for fodder (EMMER),
14 Loud music (forte = F-) is (-IS-) coming from full house perhaps (‘full house’ = hand of cards = -H AND-) with fragments (CHIPS) (FISH AND CHIPS) coming from Italian restaurant (FISH AND CHIPS),
18 Have a stab at calling him after June (July named after JULIUS CAESAR), everyone else did (JULIUS CAESER was stabbed by Brutus and Cassius, among others),
21 Berlin put out by olive branch (‘Berlin’ put out by ‘olive branch’ = ovach = HAVOC) in the utter chaos (HAVOC),
22 Reports (= homophone indicator) Hugh (“Hugh” = “hue” = HUE) Andrew (AND -Y) over credit (CR-) (HUE AND CRY) racket (HUE AND CRY),
24 What the bank doesn’t want (Blood Bank doesn’t want BAD BLOOD) is a family feud (BAD BLOOD),
25 Buries (buries = BRUISE) criminal (= anagram indicator) as a result of person getting whacked (BRUISE),
26 Burly sad (Burly sad = ABSURDLY) criminal (= anagram indicator) in a foolish sort of way (ABSURDLY),
27 Made fun of (TEASED) tenor (T-) to reduce the tension (-EASE-) – it’s key (music key -D) (TEASED).
Down:
1 Unfairly take the best (CREAM OFF) from a Merc (a merc – CREAM) convertible (= anagram indicator) – that’s bad (OFF) (CREAM OFF),
2 Cabinet (BOOKCASE) order (BOOK-) comes in front of the judge (-CASE) (BOOKCASE),
3 Turns one’s nose up at (SPURN) crazy (= anagram indicator) runs (runs = S-URN) around park (-P-) (SPURN),
5 Paddy (IRISH WHISKEY) Powers? (IRISH WHISKEY),
6 Possible (= anagram indicator) danger (danger = GARDEN) taking most of (t) organic material (‘peat’ without ‘t’ = PEA) (GARDEN PEA) from a mangetout (GARDEN PEA),
7 Mexican neighbour (USA = US A-) writing (from the 3Rs = -R-) to gracious (gracious! = My! = -MY) (US ARMY), major employer overseas (US ARMY),
8 Make certain (ENSURE) point-to-point (east + north = EN-) is on firm ground (-SURE) (ENSURE),
11 No escaping the audience here (GOLDFISH BOWL) – in first (first wins GOLD-) for The Snapper (-FISH) at amphitheatre (BOWL) (GOLDFISH BOWL),
15 Laura (Laura = A-ULAR) Cook (= anagram indicator) admits starting venture (starting ‘venture’ = -V-) at a foreign (a = -UN-) college (-C-) (AVUNCULAR) that’s helpful towards younger family members (AVUNCULAR),
16 Do they really hear if anyone’s there (PSYCHICS) or are they hearing things? (PSYCHICS),
17 Bass (= B-) – is a type of fish (-RAY) to go with top fizz (HEAD) (BRAY HEAD) in Wicklow? (BRAY HEAD),
19 Fear (PHOBIA) of hop (hop = PHO-) scotch (= anagram indicator) on rising (= reversal indicator) bank (Allied Irish Banks = AIB = -BIA) (PHOBIA),
20 Last course (last ‘course’ = E-) of order (= anagram indicator) saved (saved = -VADES) (= EVADES) by ducks (EVADES),
23 Tribunes missing bit (‘tribunes’ missing ‘bit’ = runes = NURSE) of care (NURSE).