Across
1 Finally (= position indicator i.e. second word), a good egg (EGG) turns up (GGE) with a golden (AU) heart (AU at ‘heart’ of GGE = GAUGE) and a device to measure what’s on all sides today (= PRESSURE GAUGE).
10 A mix (= AMALGAM) of endless games (GAM-) at last (= postition indicator for GAM) for the French mother (= LA MA) back there (= AMAL).
11 Refrain from commenting on (= ABSTAIN) stomach (=AB) mark (=STAIN).
12 A colourful goddess (= IRIS) to have in one’s sight (= IRIS).
13 Guide to (= USHER) Mr Poe’s badly built abode (Ref. The Fall of the House of Usher by Edgar Allan Poe), perhaps?
15 Rolled this (= DICE) around (indicates anagram) and covered the cake (ICED).
17 Black and sticky (= TAR) but I’ve heard (= homophone indicator) a second helping (i.e. ‘tartare’) goes well with fish.
19 Police (= PATROL) arranged (indicates anagram) to take cut from plutocrat (remove ‘cut’ from ‘plutocrat’ = PLORAT) .
21 Eva’s doing this (reverse ‘Eva’s’ = SAVE UP) to build reserves.
22 Bag (= SATCHEL) taken to Lech (LECH) on Saturday (SAT) by mistake (indicates anagram).
23 We must return (= EW) to start school (=SCH) in the capital of Ecuador (= E), though we might like to avoid it (= ESCHEW).
25 Sweet (= TOFFEE) account of feelings kept hidden (indicates hidden word, ‘accounT OF FEElings’).
27 Longfellow (= BOA, i.e. snake) restricts movement (ref. Boa constrictor).
29 Look to the southeast (= SE) at sunrise, perhaps, (=AM) for this layer of gold (= SEAM).
30 Wash (= RINSE) mandarin seeds inside (‘inside’ indicates hidden word, mandaRIN SEeds).
31 Responsibility felt (= ONUS) by famous star (i.e. ‘sun’) to return (reverse indicator, ‘sun’ = NUS) to ring (= O).
34 A lock of hair (RINGLET) gave Romeo (R-) a peculiar (= anagram indicator) tingle (-INGLET).
35 Oscar (= O) writes (= PEN) melody (= AIR) outside (= OPEN-AIR).
36 This might go up (= BLOOD PRESSURE) when you’re 9 down (‘under pressure’) or violently repulsed (anagram of REPULSED) by drunken boors (anagram of BOORS) (BOORS & REPULSED = BLOOD PRESSURE).
Down
2 No romanticist (= REALIST) lies about art (=anagram of LIES and ART).
3 It’s an old story (= SAGA), like the operatic start of 34 across (‘operatic Ring’, ref. Wagner’s ‘Ring’ Cycle e.g. saga), perhaps?
4 Shortly (=abbreviation), the doctors (DRS) will escape (= remove letters) the dust storm (-UTSTOM). That’s extreme (=UTMOST).
5 This might hide Hera’s error (hide hERA’S ERror)! (& ERASER literally ‘hides’ error)
6 Turn up (= reverse indicator) to take the water here (‘spa’=APS-) at the end of June (-E) in the Abbey, perhaps (APSE).
7 Carelessly (= anagram indicator) train, for example (TRAIN and EG), with something so heavy (= GRANITE).
8 Criminal Assets Bureau (= CAB) in here (= IN) puts the squeeze on high flyers (= CABIN PRESSURE).
9 It’s low down (= UNDER) to begin with (= position indicator i.e. UNDER first) coercion (= PRESSURE) from all sides (ref. ‘pressure’).
14 As idyllic and peaceful (= HALCYON) as any loch could be (= anagram, ANY LOCH).
16 Unlucky one (= LOSER) makes up (= reverse indicator) endless resolutions ( = RESOL).
18 Possible setting for ‘The Barber of Seville’ (= SALON) to be revealed (indicates hidden word) in rehearsal onstage (rehearSAL ONstage).
20 Does the Dude abide by this (= LAW) in west Los Angeles (WLA)?
21 It’s hard (= SET) to make a scene (= SET).
24 Thomas Mann loses head (= -ANN) and travels around (indicates anagram) Lech (= LECH) in a deeply groovy way (= CHANNEL).
26 The smaller university (= UNI) in the French capital (=F) may be far (=FAR) off (indicates anagram) but we’ll find some amusement there (= FUNFAIR).
27 Large garment (i.e. ‘big top’), perhaps, sometimes found at 26 down (‘funfair’) for entertainment (= BIG TOP).
28 One might find metals (= ORE) found amongst the dying embers (= ASH) on the beach (= ASHORE).
32 She sings (ALTO) the alternative (ALT) start of 34 across (‘RING’ = O)
33 See through (= LENS) the French (= LE) and the heartless Normans (remove centre of ‘Normans’ = NS).