Across:
1 Upsetting (= reversal indicator) writing (from the 3Rs = -R) by a (-A-) large (-L-) mouthy type (lip = PIL-) (PILLAR) in The Post (post/column = PILLAR),
4 Frustrated (IMPAIRED) amid ripe (amid ripe) bananas (= anagram indicator)? (amid ripe = IMPAIRED),
9 Call for (ENTAIL) at line (at line) dance (= anagram indicator) (at line = ENTAIL),
10 Edward (-NED-) is one (-I-) barman (Bachelor of Laws = -BL-) in Ireland (Ireland domain = .ie = -I-E) (INEDIBLE) not fit to serve (INEDIBLE),
12 What fiction writer is capable of (lie) for the most part (e) (‘lie’ without ‘e’ = LI-) with talent (-ABILITY) (LIABILITY)? It’s still to be settled! (LIABILITY),
13 English (E-) Times (times/multiplication symbol = -X-) has leaderless (f) column (column/row = ‘file’ without ‘f’ = -ILE) (EXILE) with a sentence that’s obviously political (EXILE / deport is a political sentence/punishment),
14 Sounds like something past master (archaeologist) would do (DIG UP THE PAST) to expose old muck (DIG UP THE PAST),
18 Trim (l) market (‘sell’ without ‘l’ = SEL-) on Friday (-F) provides attraction (INTEREST) (SELF-INTEREST) that’s to one’s personal advantage (SELF-INTEREST),
21 Boredom (ENNUI) is central to French (central to ‘frENch’ = EN-) university (National University of Ireland = -NUI) (ENNUI),
22 Performs a song (sings = S-INGS) over heads of every expectant daughter (heads of ‘Every Expectant Daughter’ = -EED-) left (-L) (SEEDLINGS) with those in The Nursery (SEEDLINGS),
24 The death centre of the farm world (ABATTOIR), and in a sense, it’s a shambles which older people will recognise (old ‘shambles’ = ABATTOIR),
25 Characters videotaped a lobster (characters ‘videotaPED A LObster’ = PEDALO) from pleasure boat (PEDALO),
26 A typical example (PARADIGM) of folding over (= reverse indicator) map (map = PA-M) covering summit on Greek (summit on ‘Greek’ = -G-) mountain on Crete (Mount Ida = Ida = -ADI-) at side of the road (at side of ‘Road’ = -R-) (PARADIGM),
27 Resign officially from organisation (SECEDE) dispersing (= anagram indicator) seed (seed = SE-DE) around Clare (-CE-) (SECEDE).
Down:
1 Bar (bar/prohibit = PRECLUDE) in park (P-) is about (-RE-) to introduce drinks (introduce 'Drinks' = -D-) in guide that appeals to cruciverbalists (clue = -CLU-E) (PRECLUDE),
2 Greatly (greatly) shaken (= anagram indicator) (greatly = LET-ARGY) consuming heroin (-H-) (LETHARGY) as it saps the energy (LETHARGY),
3 Cover story (ALIBI) to drag out of Garibaldi (‘drag’ out of ‘Garibaldi’ = ibali = ALIBI),
5 Lucrative enterprise (MONEY-SPINNER) in capital (MONEY) for evildoer (S-INNER) handling pressure (-P-) (MONEY-SPINNER),
6 Gets a package in the post (ADDRESSEE) with a letter-opener? (the one opening the letter = ADDRESSEE),
7 Bunny’s (rabbits) missing time (t) (‘rabbits’ without ‘t’ = RABBIS) with church leaders (RABBIS),
8 One of those with a reputation as a loser (DIETER) is given this name in Munich (DIETER),
11 Casting for Ray in The Dark (NIGHT FISHING) or trying to pick up the bass before it comes to light? (NIGHT FISHING),
15 Knocked back (= reverse indicator) apple drink (cider = -REDIC-) at the end of workout (end of ‘workouT’ = -T-) in gym (PE = P-E-) ahead of decathlon (‘ahead’ = a head of ‘Decathlon’ = -D) (PREDICTED) as expected (PREDICTED),
16 Captivate again (RE-ENGAGE) with a fair amount of (half of) the omen (half of ‘omEN’ = -EN-) in reggae (reggae) medley (= anagram indicator) (reggae = RE-GAGE) (RE-ENGAGE),
17 Good books (Old Testament = OT-) on top (‘On’ top = -O-) Garda (-COP-) in southeast (-S-E) (OTOSCOPE) – the one investigating what’s going on along the canal (OTOSCOPE),
19 It’s about (RE-) The British Museum (Victoria & Albert Museum/ V&A = -VA-) contacting foreign politician (-MP) (REVAMP) with update (REVAMP),
20 It’s not just (it’s UNFAIR) one-sided? (UNFAIR),
23 Fiction (LIE-), for example (e.g.), upsets (= reversal indicator) (e.g. = -GE) (LIEGE) one of those in Belgium (LIEGE).