Across:
1 What one exercising strong-arm tactics produces perhaps (BICEPS) in seafood (mussels) report (= homophone indicator) ('mussels' = 'muscles' = BICEPS),
4 Those in Mayo (those in mayonnaise = EGG YOLKS), for example (eg = EG-), and those around Galway (those around ‘GalwaY’ = -G Y-) people (folks), need no introduction (f) (‘folks’ without ‘f’ = -OLKS) (EGG YOLKS),
9 Opens 14 across (opens ‘Greek Islands’ = GREEK-) for Pole (South = -S) (GREEKS) living in Athens (GREEKS),
10 Usual fob (usual fob) off (= anagram indicator) (usual fob = FABULOUS) is phenomenal (FABULOUS),
12 Not hurt (UNSCATHED) by a foreign (‘a’ foreign/French = UN-) lawyer (Senior Counsel = -SC-) – a (-A-) man (-HE-) taken in by politician (Teachta Dála = TD = -T-D) (UNSCATHED),
13 Flexible (AGILE) barman (Attorney General = AG-) blows top (f) in row? (row/line = ‘file’ without ‘f’ = -ILE) (AGILE),
14 European (GREEK) school (-S) behind (= position indicator) 25 across (‘island’ = ISLAND-) (GREEK ISLANDS) in the Mediterranean (GREEK ISLANDS),
18 Contests (COMPETITIONS) firm (CO-) is getting 1000 (-M-) formal requests with signatures (-PETITIONS) (COMPETITIONS),
21 Sets up (= reverse indicator) German newspaper (Bild = -D LIB) to answer (A-) (AD LIB) what’s been said off the cuff (AD LIB),
22 One of those perhaps responsible for 24 across? (AEROPLANE responsible for ‘sky lines’) Presumably you heard that from Sky! (AEROPLANE),
24 Views of The Tower (SKYLINES) and South (S-) Kerry (-KY-) bars (-LINES) (SKYLINES),
25 Most (t) of the bias (slant) (‘slant’ without ‘t’ = -SLAN-) in papers (ID papers = I-D) (ISLAND) in Ireland, for example (ISLAND),
26 Alter the picture (AIRBRUSH) and melody (AIR-) for supporting film (-B- film) shoot, in a dashing way (-RUSH) (AIRBRUSH),
27 Leased (leased) complex (= anagram indicator) (leased = SEALED), as it’s yet to open (SEALED).
Down:
1 Resent (BEGRUDGE), for example (eg = -EG-), never-ending ('neveR' ending = -R-) shift (budge = B-UDGE) outside (BEGRUDGE),
2 Ship (-S S-) close to Salé (close to ‘salE’ = -E-) consumed by Pirate’s strongbox (chest = CHES--T) (CHESS SET) containing pieces that will appeal to those who want a mate? (CHESS SET),
3 Genuine local (PUKKA) pressure (P-) on neighbours (UK = -U-K-) over weekend (‘weeK’ end = -K-) to take top advice (top ‘Advice’ = -A) (PUKKA),
5 One thousand (GRAND-) teachers (-MASTERS) (GRANDMASTERS) similar to those in 11 down but better (GRANDMASTERS similar but better ‘chess players’),
6 I completely agree (YOU SAID IT) with what’s declared (SAID) after (= position indicator) hearing (= homophone indicator) one of the flock (‘ewe’ = ‘you = YOU) lost the head (w) with comedian (‘wit’ without ‘w’ = - IT) (YOU SAID IT),
7 Pay a quick visit to (LOOK IN) gent’s (gents = LOO-) family (kin = -K IN) (LOOK IN),
8 Relative (SISTER) removes wall from stairwells (removes ‘wall’ from ‘stairwells’ = stires = SISTER),
11 Those associated with the board bound to make a move (CHESS PLAYERS) by opening 2 down (opening ‘chess set’ = CHESS) and 20 down (‘player’ = PLAYER-) at the start of season (start of ‘Season’ = -S) (CHESS PLAYERS),
15 To the eye, the lumpier part (KNOBBLIER) of Kreuzberg leading (‘Kreuzberg’ leading = K-) to river (River Ob = -OB-) in Berlin (Berlin) is wild? (= anagram indicator) (Berlin = -N-BLIER) (KNOBBLIER),
16 Don’t mention it (NOT AT ALL) to a heavyweight (a ton) turning up (= reversal indicator) (a ton = NOT A) high (tall = -T ALL) (NOT AT ALL),
17 A (A-) perfume (scent) that’s endless (t) (‘scent’ without ‘t’ = -SCEN-) from heads of decorative, elegant damask (heads of ‘Decorative Elegant Damask’ = -DED) (ASCENDED) rose (ASCENDED),
19 It’s part of the plot (CASSIA tree/shrub) in Isaac’s (Isaacs) novel (= anagram indicator) (Isaacs = CASSIA),
20 Layperson has no son (‘layperson’ has no ‘son’ = layper = PLAYER) working in the abbey (working in The Abbey Theatre = PLAYER),
23 Some revolutionary (= reverse indicator) business approach (some ‘businESS APproach’ = essap = PASSE) is no longer fashionable (PASSE).