Across:
1 School leaves psychologist (‘school’ leaves ‘psychologist’ = pygist = PIGSTY) in a mess, figuratively speaking (PIGSTY),
4 It’s weird (ODD) to crawl out of bed (get up = GETUP) (ODD GETUP) in strange robes (ODD GETUP),
9 Smells (ODOURS) funny? (= anagram indicator) Ours do! (ours do = ODOURS),
10 A shelter (a shelter) compromised (= anagram indicator) (a shelter = LEATHERS) by Hells Angels outfit? (LEATHERS),
12 Tweet (PEEP-) from those on the golf course (-HOLES) (PEEPHOLES) giving some insights into how those in The Houses secretly see things (PEEPHOLES),
13 Some nutmeg rustled (some ‘nutmEG RUStled’ = egrus = SURGE) up (= reverse indicator) in Rush (SURGE),
14 Issues in The Palace (PRINCES) following (= position indicator) 23 down? (‘crown’ = CROWN) (CROWN PRINCES) And, they expect them to succeed! (CROWN PRINCES),
18 Town in Wicklow (AVOCA-) and Hull (shell/pod) raising (= reverse indicator) (pod = -DO P-) units of corn (-EARS of corn) (AVOCADO PEARS) and a number of fruits (AVOCADO PEARS),
21 Lodging house (hotel) removes top on table (top on ‘Table’ = ‘t’) and replaces it primarily with veneer (primarily with ‘Veneer’ = ‘v’) (remove ‘t’ from ‘hoTel’ and replace it with ‘v’ = HOVEL) from The Shack? (HOVEL),
22 How locals (how locals) evolve (= anagram indicator) (how locals = LAW SCHOOL) for the barman of the future (LAW SCHOOL),
24 Trick of the light (ILLUSION) in castles in Spain? (ILLUSION),
25 Broadcasting (-ON-) in The Cloud (mist = M-IST) (MONIST) by single-minded philosopher (MONIST),
26 Lay down (KEEP-) type of wine (-SAKE rice wine) (KEEPSAKE) for the memorial (KEEPSAKE),
27 Backs (= reverse indicator) model (sit) for the most part (t) (‘sit’ without ‘t’ = -IS) on vocal composition (song = GNOS-) (GNOSIS) with knowledge of spiritual mysteries (GNOSIS).
Down:
1 Pops cert (pops cert) out (= anagram indicator) (pops cert = PROSPECT) in anticipation (PROSPECT),
2 Legend (‘leG’ end = G-) gets nothing from tennis (-LOVE-), from fight in the ring (-BOX in the boxing ring) (GLOVEBOX) or from one of the handy types in golf? (handy type in VW Golf car = GLOVEBOX),
3 Olympic symbol (TORCH) held by Francie Barrett or cheerleader (held by ‘francie barretT OR CHeerleader’ = TORCH)
5 Lecture (DRESSING DOWN) for putting on a uniform perhaps (DRESSING) in Northern Ireland (DOWN) (DRESSING DOWN),
6 Retire (GO TO SLEEP), as it’s too (too) bad (= anagram indicator) (too = -O TO) with endless (t) driving hazard (sleet) (‘sleet’ without ‘t’ = SLEE-) in Grand Prix (G--P) (GO TO SLEEP),
7 Hypothesis (THEORY) leads to extremes? The rest is history! (extremes of ‘THE rest is histORY’ = THEORY),
8 A concern for historians (PAST-) taking the Spanish (‘the’ Spanish = -EL) (PASTEL) shade (PASTEL),
11 Jobs in The Office (CLERICAL WORK) for those tending to the flock (CLERICAL WORK),
15 Writer’s (writers = NOVELISTS) romance (NOVEL-) is (-IS-) initially tense (initially tense = -T-) and close to tempestuous (close/end to ‘tempestuouS’ = -S) (NOVELISTS),
16 Most of (t) the component (‘part’ without ‘t’ = PAR-) from Oxford (-O-) stops working (-DIES) (PARODIES) with apes (apes/skits = PARODIES),
17 Fences off (ISOLATES) aisles to (aisles to) building (= anagram indicator) (aisles to = ISOLATES),
19 Horticultural society overseas (RHS) set-up (= reverse indicator) (RHS = SHR-) on popular (-IN-) weekend (‘weeK’ end = -K) (SHRINK) in retreat (SHRINK),
20 Revelations released on air (‘revelations’ released ‘on air’ = evelts = SVELTE) in Trim (trim/slender = SVELTE),
23 Top (CROWN) ground crew urged to take off (‘ground crew’ urged to take off /without ‘urged’ = oncrw = CROWN).