Across:
1 Breaks (= anagram indicator) code (code = DECO-) by reading (from the 3Rs = -R-) introduction to schematic (introduction to ‘schematic’ = -S) (DECORS) and looks inside the house (DECORS),
4 Well-trained (OBEDIENT) cook (= anagram indicator) on diet (on diet = O-DIENT) welcomes honey producer (bee) for the most part (e) (‘bee’ without ‘e’ = -BE-) (OBEDIENT),
9 Relative (MA-) from Copenhagen (Dane = -ENAD) upsets (= reversal indicator) (MAENAD) wild woman from Greece (MAENAD),
10 Ulster (U-) bus lane (bus lane = -NUSABLE) (UNUSABLE) potentially is not fit for purpose (UNUSABLE),
12 Tamara (-TAMARA-) is found in container (CA-N) (CATAMARAN) in sea-going craft (CATAMARAN),
13 The European (the = EL-) contacts the German (the = -DER) (ELDER) church official (ELDER),
14 Tree (ASH) named after god of victory? (WEDNESDAY named after Odin/Woden/Wotan, the god of victory and death) (ASH WEDNESDAY) Time to give up! (ASH WEDNESDAY, the start of Lent),
18 Community (SOCIAL) on hand (WORKER) (SOCIAL WORKER) looking after the welfare of others (SOCIAL WORKER),
21 Worry (CAR-E) about sign (-V- sign) (CARVE) for a job in the butchers (CARVE),
22 Approach (STYLE-) in newspaper section (Arts = ART -S) (ART STYLES) leads to symbolism and realism (ART STYLES),
24 Champagne and German wine (both types of RHUBARBS)? They’re a bit stalky for dessert (RHUBARBS),
25 Shopkeeper will thank you for it (CUSTOM) – it’s traditional (CUSTOM),
26 Incidentally (BY THE BYE), who wrote this (BY) article (THE) on what the batsman got for missing it? (cricket BYE) (BY THE BYE),
27 Is it right (R-) to have a German (a = -EIN) in (IN) (REIN IN) control? (REIN IN).
Down:
1 Taster of The Band (DEMO-) at college (-C-) tends to inform (-RAT) (DEMOCRAT) an advocate of house party (an advocate of The House in the political party/government and democracy = DEMOCRAT),
2 Those fast, wild types (CHEETAHS) produced (= anagram indicator) The Chase (The Chase = CHEETAHS),
3 It’s about (RE-) Ulster politician (MLA = -ALM) retiring (= reversal indicator) (REALM) to The Kingdom (REALM),
5 Dinner for the swingers (BANANA) in Croatia (SPLIT-) with small (-S) (BANANA SPLITS) sweet treats, of course (BANANA SPLITS),
6 Padre does (Padre does = DESPERADO) badly (= anagram indicator) with reckless villain (DESPERADO),
7 Closes the mechanism (closes ‘the mechanism’ = EM-) and frame (-BODY) (EMBODY) to take in (EMBODY),
8 Speculation (THEORY) by several banks? The same old story! (several banks of ‘the same old story’ = THEORY),
11 Standard (“principal” = “PRINCIPAL”) report (= homophone indicator) by one of the lads (BOY) (PRINCIPAL BOY) and one of the women taking lead in a pantomime (PRINCIPAL BOY usually played by a female),
15 Absolve (EXONERATE) old partner (EX-) losing the head with (t) sound quality (‘tone’ without ‘t’ = -ONE-) in front of (= position indicator) judge (-RATE) (EXONERATE),
16 The bare bones (SKELETON) of novel (= anagram indicator) Ken stole (Ken stole = SKELETON),
17 Mill (PRESS-) worker (-MAN) (PRESSMAN) has a job making a long story short perhaps (journalist/sub-editor = PRESSMAN),
19 Sounds like (= homophone indicator) one of The Beatles (one of the Beatles = “Beatle” = “beetle” = SCARAB) is lucky and charming perhaps (SCARAB is a lucky charm),
20 Break in (IRRUPT) – it (I-T) sounds like car (purr = -RRUP-) reversing (= reversal indicator) inside (IRRUPT),
23 Telling it like it is (TRU-E) about Charlie (-C-) (TRUCE) – no argument there for the time being (TRUCE).