Across:
1 Deal (PAC-T) covers short (y) codebreaker (‘key’ without ‘y’ = -KE-) (PACKET) with a large sum of money (PACKET of money),
4 Transferred knowledge (UPLOADED) from university (U-) quietly (soft/softly = piano = -P-) for lots of money (-LOADED) (UPLOADED),
9 Names a (names a = SEAMAN) criminal (= anagram indicator) working in Mediterranean, for one (SEAMAN),
10 Current (flow = WOLF) review (= reversal indicator) with introductions covering unsurprisingly Bogart, Sinatra (introductions ‘covering unsurprisingly Bogart, Sinatra’ = CUBS) (WOLF CUBS) and newer members of The Pack (WOLF CUBS),
12 Halted car (halted car = CATHEDRAL) travelling (= anagram indicator) to place of mass meeting (CATHEDRAL),
13 Local car (wheels) fails to start (w) (‘wheels’ without ‘w’ = HEELS) with parts from the boot (HEELS),
14 This is so obvious (DEAD GIVEAWAY) to Will? (a will = DEAD GIVEAWAY),
18 A little drunk (MERRY -) gets green light (- GO-) to get drinks for everyone (-ROUND) (MERRY-GO-ROUND) – it creates amusement (MERRY-GO-ROUND),
21 Heard (= homophone indicator) course (“course” = “coarse” = HARSH) is rough enough (HARSH),
22 Has a bone to pick (BEEF-) with consumer (-EATER) (BEEFEATER) working in security overseas at The Tower (BEEFEATER warden/guard in The Tower of London),
24 Home- (IN-) -town leader (‘town’ leader = -T-) is cheeky (-RUDE-) writing (from the 3Rs = -R) (INTRUDER) to one of those breaking into the business (INTRUDER),
25 Dean leaves broad beans (“Dean” leaves “broad beans” = broabs = ABSORB) to soak (ABSORB),
26 Awful (= anagram indicator) despair (despair = PAR-DISE) over a (-A-) (PARADISE) form of ecstasy (PARADISE),
27 Waitress loses it (‘waitress’ loses ‘it’ = WRASSE) with bright fishy type (WRASSE).
Down:
1 Support (support/pillar = POST-) for a set of standards (-CODE of standards) (POSTCODE) produced by Dublin fifteen, for instance? (Dublin 15 = POSTCODE),
2 Applause (CLAP-) for trick (-TRAP) (CLAPTRAP) is absurd (CLAPTRAP),
3 Some revolutionary (= reverse indicator) characters mentioned a vendetta (characters ‘mentioned a vendetta = edave = EVADE) to escape (EVADE),
5 Made a bad case (POORLY ARGUED) for most of (t) the drugs (‘pot’ without ‘t’ = PO-) found at Paris airport (-ORLY airport) and tried to defend it? (ARGUED) (POORLY ARGUED),
6 Not being capable of a tune (OFF THE AIR) is not on the radio (OFF THE AIR),
7 Flipping (= reversal indicator) source (seed = DE-ES) protects copper (cu = -UC-) (DEUCES) – they’re two of a kind (DEUCES),
8 Is there no wine in West Indies (no ‘wine’ in ‘West Indies’ = estdis = DESIST)? Give up! (DESIST),
11 It’s crucial (GRAVE) for shoplifters (ROBBERS) (GRAVE ROBBERS) and those on the take, of late (GRAVE ROBBERS),
15 Goes to the dogs (GREYHOUND racing) with American coach (GREYHOUND bus/coach),
16 Those selling old wines, for instance (AUCTIONS), to those who have lots presumably (AUCTIONS),
17 Precious type (ADORABLE) of hammer (= anagram indicator) or a blade (or a blade = ADORABLE),
19 Sister (Sr = S-R-) protects top of head (top of ‘head’ = -H-) from devilish creature (-IMP) (SHRIMP) swimming in the sea (SHRIMP),
20 Warrant (WRIT-) for that woman (her) losing the head (h) (‘her’ without ‘h’ = -ER) (WRITER) reported in the paper (WRITER),
23 Who is out of homebrew (‘who’ is out of ‘homebrew’ = mebre = EMBER) and the last of the blackstuff? (EMBER).