Across:
1 Turns up (= reverse indicator), relay (relay) brief (y) (‘relay’ without ‘y’ = rela = -ALER) by journalist (ed = DE-) (DEALER) to one selling drugs (DEALER),
4 Donnybrook’s (Donnybrooks = SCUFFLES) small (S-) bash (-CUFF-) for the French (the = -LES) (SCUFFLES),
9 Takes up room (LODGER) in someone else’s house (LODGER),
10 Puts up a fight defending an alternative opinion (COUNTERS) dividing the customers and publicans (COUNTERS),
12 Funny (= anagram indicator) cartoon (cartoon = CONTRA-O) includes soldier (lieutenant = -LT-) (CONTRALTO) with singing voice (CONTRALTO)
13 Tuned in (AWARE) to On the Ball (AWARE),
14 Peers (COUNTERPARTS) in 10 across (counters) for the most part (s) (‘counters’ without ‘s’ = COUNTER-) with roles to play (-PARTS) (COUNTERPARTS),
18 Brief (s) 10 across (‘counters’ without ‘s’ = COUNTER-) and most of (s) 25 across (‘claims’ without ‘s’ = -CLAIM) (COUNTERCLAIM) for a different response (COUNTERCLAIM),
21 Motoring organisation (AA = A-A) welcomes Pat (pat = rub = -RUB-) (ARUBA) to island in the Caribbean (ARUBA),
22 Finish off (ERADICATE) a cider (a cider) cocktail (= anagram indicator) (a cider = ERADIC-) and put away (-ATE) (ERADICATE),
24 Those enlisted (SOLDIERS) in the pay of a major employer (SOLDIERS),
25 Protest (CLAIM-) by Southern (-S) (CLAIMS) States (states/asserts = CLAIMS),
26 Miser isn’t (miser isn’t) recycling (= anagram indicator) (miser isn’t = MINISTERS) contents of cabinet (MINISTERS),
27 Making contact with distributor (POINTS) for new (= anagram indicator) piston (piston = POINTS).
Down:
1 Sensitivity (DELICACY) to gourmet food? (DELICACY),
2 Rider (rider/add-on = ADDENDUM) is one for the books (ADDENDUM),
3 Turns up (= reversal indicator) in agreement (in ‘agREEMEnt’ = reeme = EMEER) with Arab leader (EMEER),
5 Dark brown (CHOCOLATE brown) shaft (BAR) (CHOCOLATE BAR) from Milky Way (CHOCOLATE BAR),
6 It’s implausible (FANTASTIC) for fat cats in (fat cats in) compound (= anagram indicator) (fat cats in = FANTASTIC),
7 Provides free rein (LEEWAY) for river (River LEE-) approach (-WAY) (LEEWAY),
8 Organisation (SYSTEM) produces mass hysteria dismissing Sharia (‘mass hysteria’ dismissing ‘Sharia’ = mssyte = SYSTEM),
11 Obvious mistake (GLARING ERROR) with dazzling light (GLARING light) starting off (t) panic (‘terror’ without ‘t’ = ERROR) (GLARING ERROR),
15 Occupied (TOOK) by answer (A) to clue? (HINT) (TOOK A HINT) It’s what every successful cruciverbalist did, advisedly! (TOOK A HINT),
16 German (BAVARIAN) voices introduce (‘Voices’ introduce = -V-) bit of Puccini, for example (-ARIA-), in bar (BA-N) (BAVARIAN),
17 Odd (= anagram indicator) series (series = I-ERSES) about Frenchmen (Messieurs = -MM-) (IMMERSES) goes down overseas like a lead balloon perhaps (IMMERSES),
19 Attractive (handsome) broadcast (= homophone indicator) (‘handsome’ = ‘HANSOM’) for one getting drawn along by the gee-gees (HANSOM),
20 Yank (PULL) is not at home (IN) (PULL IN) to arrest (PULL IN),
23 Soldier (GI) returns (= reversal indicator) (GI = IG-) from centre overlooking (centre ‘overLOOking’ = -LOO) (IGLOO) residence way up north (IGLOO).