Across:
1 Welsh politician (-MP-) consumed by devious plan (wile = WI-LE) (WIMPLE) that goes over the novice’s head (WIMPLE worn by novice /nun),
4 A (A-) hit (-BASH-) in (-IN-) Government (-G) (ABASHING) – that’s embarrassing? (ABASHING),
9 No sermons in Expressionism (no ‘sermons’ in ‘Expressionism’ = expsii = PIXIES) from the mischief-makers (PIXIES),
10 CAB (cab = TAX-I-) conceals a (-A-) final benefit (final ‘benefit’ = -T-) on (-ON) (TAXATION) raising State’s finances (TAXATION),
12 Sister (DOMINICAN sister/nun) in nomadic (in nomadic = DOMINICAN) settlement (= anagram indicator),
13 One of the joints (KNEE-) left (-L) (KNEEL) to church movement (KNEEL),
14 Make Istanbul (make Istanbul = UNMISTAKABLE) out (= anagram indicator) to be distinctive (UNMISTAKABLE),
18 Measure up (MAKE THE GRADE) model (MAKE) with definite (definite article = THE) degree of proficiency (score/mark = GRADE) (MAKE THE GRADE),
21 Reasonable (“sane” = “Seine” = SEINE) report (= homophone indicator) divides Paris (SEINE),
22 Wind up (ANNOY-) Spike (lance) losing the head (s) (‘lance’ without ‘s’ = -ANCE) (ANNOYANCE) with nuisance (ANNOYANCE),
24 Drunk in France (CALVADOS) gets locked first (‘locked’ first = -L-), tucking into Spanish wine (Cava = CA-VA-) at parties (parties/do’s = -DOS) (CALVADOS),
25 Boatman leaves Guantanamo Bay (‘boatman’ leaves ‘Guantanamo Bay’ = guanay = GUYANA) for country in South America (GUYANA),
26 Compensate for (OUTWEIGH) women (-W-) from Aer Lingus (-EI-) in tough (tough = OUT-GH) flight (= anagram indicator) (OUTWEIGH),
27 Memo (AGENDA) from head-banger dismisses Herb (‘head-banger’ dismisses ‘herb’ = adange = AGENDA).
Down:
1 Eliminated (WIPED OUT) model’s (models = anagram indicator) wide pout (wide pout = WIPED OUT),
2 The most you’ll get (MAXIMUMS) for graduate (MA-) with eleven (-XI-) family members (-MUMS) (MAXIMUMS),
3 Stay under the covers (LIE-IN) finishing (= positional indicator) popular (IN) fiction (LIE), (LIE-IN),
5 Mind (BRAIN-) – one might have to be hung out to dry after this (-WASHING) (BRAINWASHING), as it’s part of the conditioning (BRAINWASHING),
6 Firing device found in saloon (car) (SPARK PLUG) in small (S-) green area (PARK) in Bath (PLUG / stopper in bath) (SPARK PLUG),
7 One (I-) offer at the auction (-BID-) for half the object (half the ‘item’ = -EM) (IBIDEM) in the same place (IBIDEM),
8 Kind of person (GENTLE) one leaves (i) outside religious group (‘gentile’ without ‘i’ = GENTLE),
11 Just about managed (SCRAPED ALONG) without pressure (p) – scrapped (‘scrapped’ without ‘p’ = SCRAPED) Anglo (Anglo = ALONG) Review (= anagram indicator) (SCRAPED ALONG),
15 Gone off (STALE-) position on ship (-MATE) (STALEMATE) – it’s the dead heat? (dead heat/draw = STALEMATE),
16 Keep up (MAINTAIN) with important (MAIN-) Irish legend (Táin = -TAIN) (MAINTAIN),
17 Lost (= anagram indicator) a dreamer (a dreamer = DEMERARA)? Sugar! (DEMERARA sugar),
19 Nutcase (PSYCHO) begins post seminary year (starts ‘post seminary year’ = PSY-) at church (-CH-) order (-O) (PSYCHO),
20 Remove bones from fish (FILLET) or piece of meat (FILLET),
23 Only one of us (one of ‘you and me’ = YOU-) gets a bad result (no grade = -NG) (YOUNG) in the early years (YOUNG).