Across:
1 Weekend (‘weeK’ end = -K-) in revolutionary (= anagram indicator) Cuba (Cuba = BAC- U-) playing the lead (‘Playing’ the lead = -P) (BACK UP) or support? (BACK UP),
4 House warmer in The Underworld (HELLFIRE) for two-thirds of 18 across (two-thirds of ‘HELL FIRE club’ = HELLFIRE),
9 It’s well (OIL RIG) for those doing the boring stuff (OIL RIG),
10 A (A-) VIP’s (vips) problem (= anagram indicator) (vips = -SP VI-) upsets (= reverse indicator) agent (rep = -PER) (ASP VIPER) – a real snake named after two crawlers? (ASP VIPER),
12 Lover (HONEY-) from Hook (-TRAP) (HONEYTRAP) and The Sting with its seductive proposition for a gent? (‘a gent’ = ‘agent’) (HONEYTRAP),
13 Father (SIRE-) last seen (last ‘seeN’ = -N) (SIREN) and heard coming from a Garda car (SIREN),
14 The type of place that prepares devils on horseback (HELL’S KITCHEN) for an old New York suburb? (HELL’S KITCHEN, Manhattan),
18 First four characters from 14 across (first four characters in ‘HELLs kitchen’ = HELL) get sack (FIRE) from bar (CLUB) (HELL FIRE CLUB) in an old gentlemen’s club with a Mephistophelian reputation (HELL FIRE CLUB, Mont Pelier Hill, Dublin),
21 In retrospect (= reversal indicator), two coppers (Detective Inspector + copper metal = DI + Cu = dicu = -UCID) left (L-) (LUCID) sober (LUCID),
22 Real tug or (real tug or) foul? (= anagram indicator) (real tug or = REGULATOR) That’s a job for someone supervising the rules! (REGULATOR),
24 Bolt (U- bolt) for safety mechanism (safety pin = -P IN) on guns (ARMS) (UP IN ARMS) when at boiling point? (UP IN ARMS),
25 There is no spa in Singapore? (there is no ‘spa’ in ‘Singapore’ = ingore = IGNORE) Never mind! (IGNORE),
26 A German (ein = E-IN-) swallowing port (flue/outlet = -VENT-) is good (-G) (EVENTING) sport (EVENTING),
27 What you’d expect 11 down to do (expect ‘artillerymen’ to ATTACK) to give someone a bad press (ATTACK).
Down:
1 Makes a great fuss (BROUHAHA) of brother (BR-) where French (French ‘where’ = -OU-) have a couple of laughs (-HAHA) (BROUHAHA),
2 Playing (= anagram indicator) on cellos (on cellos = COLONELS) or working on the base? (COLONELS),
3 Scotland is not part of dysfunctional (‘Scotland is not part of ‘dysfunctional’ = yfuni = UNIFY) link-up (UNIFY),
5 It’s not difficult (EASY) to take selfie (PIC-) with royals (-KINGS) (EASY PICKINGS) – the ones that effortlessly fall into your lap (EASY PICKINGS),
6 The hot shot presumably (LOVE SCENE) from Romeo and Juliet? (from Romeo and Juliet = LOVE SCENE),
7 Source from abroad (IMPORT) opens intoxicating Merlot (opens ‘Intoxicating Merlot’ = IM-) wine (-PORT) (IMPORT),
8 Bartender loses bet (‘bartender’ loses ‘bet’ = arnder = ERRAND) – in the run where someone is expected to collect (ERRAND),
11 Those gunning (ARTILLERYMEN) for those in 2 down, for instance (‘colonels’ = ARTILLERYMEN),
15 Dedicated (STEADFAST) to odd defeat (odd letters in ‘defeat’ = -DFA-) in States (states = STEA-ST) all over the place (= anagram indicator) (STEADFAST),
16 It means a great deal (PLETHORA) to domestic worker (help) retiring (= reverse indicator) (help = PLE-H) over square (-T-) or (-OR-) close to the sea (close/end to ‘seA’ = -A) (PLETHORA),
17 Belgium (-BE-) Royal (-R-), for example (e.g. = -GE-), backing up (= reverse indicator) Earl (-E-) in neighbouring country (UK = U-K) (UBER GEEK) – the one in the best anorak (UBER GEEK),
19 Savages (claws) state (= homophone indicator) (‘claws’ = ‘CLAUSE’) for what’s included in someone’s sentence (CLAUSE),
20 Old Jewish theologian (SCRIBE) is working on 19 down, for instance (SCRIBE is working on ‘clause’ in a document),
23 The Sun is responsible for this (LIGHT) insult (slight) dismissing leader off (s) (‘slight’ without ‘s’ = LIGHT).