Across:
1 Seems a (seems a) mess (= anagram indicator) (seems a = SESAME) or the seed of one? (SESAME seed),
4 Remove half of them from Number 10 (TAKE FIVE from the number 10) and relax (TAKE FIVE),
9 It’s utter madness (LUNACY) as there’s no greener part of Nuclear Energy (as there no ‘greener’ part of ‘nuclear energy’ = nuclay = LUNACY),
10 Boss (= anagram indicator) in Sparta (in Sparta = ASPIRANT) has ambitions (ASPIRANT),
12 “Copper (CU-) is not (not) overly (= reverse indicator) not = -T ON) depressed (DOWN) inside” (CUT DOWN ON) – Shrink (CUT DOWN ON),
13 Praise (EXALT) for latex (latex) creation (= anagram indicator) (latex = EXALT),
14 Short memo (note = NOT -E) about endless (o) teachers (Irish National Teachers’ Organisation = ‘INTO’ without ‘o’ = IN T-) heading for horrible (heading for ‘Horrible’ = -H-) dire destiny (doom) over (= reverse indicator) (doom = MOOD) (NOT IN THE MOOD) lack of enthusiasm (NOT IN THE MOOD),
18 Address (PRESENTATION) of the sisters (PRESENTATION Sisters),
21 Painting (IMAGE) of Frenchman (Monsieur = -M-) at the bar (Attorney General = -AG-) in Ireland (Ireland domain = .ie = I-E) (IMAGE),
22 Parish (AREA) hieroglyphics (CODES) (AREA CODES) with some numbers synonymous with regional figures (AREA CODES),
24 Is Tracy’s (is Tracy’s) butcher (= anagram indicator) (is Tracy’s = SACRISTY) found close to the church? (SACRISTY),
25 School (S-) friend from Ireland (Irish ‘friend’ = ‘cara’ = -CARA-) with Black (-B) (SCARAB) Beetle (SCARAB),
26 The House (HOME-) is (is) upset (= reversal indicator) (is = -SI-) by a fair amount (fair amount/half of) of the pack (half of ‘paCK’ = -CK) (HOMESICK) – could be a pining to go to the country (HOMESICK),
27 A French department (VENDÉE) store’s buyer? (VENDEE).
Down:
1 Lies in (lies in) circulation (= anagram indicator) (lies in = SILI-NE) about firm (-CO-) (SILICONE) producing material for plumbers (SILICONE),
2 Tiny cast (tiny cast) touring (= anagram indicator) (tiny cast = SANCTITY) with A Life of Holiness? (SANCTITY),
3 Sample of Hiroshima chopsticks (sample of ‘hiroshiMA CHOpsticks’ = MACHO) produced by Butch (MACHO),
5 Dreaming (ABSENT-MINDED) of not being at work? (ABSENT-) Guard (-MIND-) and writer! (-ED) (ABSENT-MINDED),
6 Reviews (= reverse indicator) short (g) journal (log) (‘log’ without ‘g’ = lo = -OL-) among all the letters (A-Z = Z-A) by Earl (-E) Green (lime = EMIL-) (EMILE ZOLA) – a famous writer with a Nana he wrote about? (EMILE ZOLA wrote novel “Nana”),
7 To a certain extent (IN A WAY), it’s fashionable (IN) on holiday? (‘away’ = ‘a way’ = A WAY) (IN A WAY),
8 Concerns Will (ESTATE) and sweetheart (‘swEet’ heart = E-) from New York (-STATE) (ESTATE),
11 Warm (ENTHUSIASTIC) with fan (ENTHUSIAST-) when one (-I-) is cold (-C) (ENTHUSIASTIC),
15 Chase (run) is back (= reverse indicator) (run = NUR-) – it’s a regular show (-SERIES) (NURSERIES) where numerous issues can be raised (numerous issues/children in NURSERIES),
16 Beastly type (female deer = HIND-) on drugs (ecstasy = -E-) studied (read) report (= homophone indicator) (‘read’ = ‘red’ = -RED) (HINDERED) to put up barriers (HINDERED),
17 Ulster (U-) bus lane (bus lane) potentially (= anagram indicator) (bus lane = -NUSABLE) (UNUSABLE) is not fit for purpose (UNUSABLE),
19 Gear (KIT-) for half the educational institute (half of ‘SCHool’ = -SCH) (KITSCH) is garishly in poor taste (KITSCH),
20 Gangster (-AL- Capone) followed by copper (-CU-) consumed by thulium (Tm = T-M) (TALCUM) – a white powdery substance (TALCUM powder),
23 Seasonal work (CYCLE) for the pedlars (pedlars) in a manner of speaking (= homophone indicator) (for the ‘pedlars’ = for the ‘pedallers’ = CYCLE).