Approximates by Adam

Gregson immediately thought of Procrustes after reading the preamble of this week's puzzle by Adam but then struggled to make much initial headway since the method or methods of entry seemed to have nothing in common with those of Procrustes, and so it turned out. Answers which were too long had all duplicated letters removed before entry. Answers which were too short had duplicated letters added and were then jumbled before entry.

The title seems rather odd, since Procrustes forced his victims to fit his bed exactly, regardless of whether they were an approximate fit or not to start with, and the same seems to be true here.

In the following explanations, answers which are too short or too long for their lights are followed by the corresponding grid entry and extra letters indicated by wordplay are shown in square brackets.

ACROSS
1 Mature venison cooked with traces of parsley, thyme and dill is not revealing much (12)
UNDEMONSTRATIVE[P] (anag of MATURE VENISON + PTD)
9 PM down under delivered in clear voice (4)
ARVO (hidden)
10 Newborn babies who are seen in a gallery? (7)
STRANGERS [STRANGE] (ref to Strangers' Gallery)
12 Most precise prohibitionists have it (6)
DRIEST (DRIES (prohibitionists) + T (it))
13 There's thin mud in the water channel (6)
SLU[R]IT [ULITIS] (SLUR (thin mud) + I (in) + T (it))
14 It's large, shaped like insect's back, perhaps (8)
TERGAL [EGLATERE] (T (it) + anag of LARGE)
16 A low scoring ball decided (5)
ARED [DREAR] (A + RED (low scoring ball)) Perhaps a bit unfair on solvers from countries where snooker is not played
17 It's good to abandon a fish spear (5)
GLEAVE [GLEAV] (G (good) + LEAVE (abandon))
18 It's rare to carry away, a good fellow (6)
ASPORT (A + SPORT (good fellow))
21 A corpse seen when rebuilding side of ditch (6)
ESCARP[O] (anag of A CORPSE)
23 It would be capital for mother to take in this plant (5)
ANIL [LANAI] (MANILA (capital) = MA (mother) round ANIL (plant))
25 Mountain bird gathers a fish (5)
TWAITE [TWAIE] (TWITE (mountain bird) round A)
29 Active university request to reader to go on and satisfy (8, three words)
LIVE UP TO (LIVE (active) + U (university) + PTO (request to reader to go on))
30 Tart? One's last two pieces eaten (6)
[C]HARLOT (CHARLOTTE (tart) minus last two letters)
31 One to restore amends gets the bird (6)
ROBIN [INBORN] (ref Robin Goodfellow, AMND Act v Sc i) Rather vague reference to a quote which does not appear in any of Gregson's dictionaries
32 Eg, writing in this language would be uplifting (7)
LATIN [ITALIAN] (ELATING (uplifting) = EG round LATIN (language)) A neat and misleading transformation of one language into another
33 Weep with banality when he's excluded in some circles? (4)
C[R]YCLIC [CYLI] (CRY (weep) + CLICHÉ (banality) minus HE)
34 A student after college's insincere being made of many elements (12)
POLYSYNTHETICAL [POLYSNTHEICA] (POLY (college) + SYNTHETIC (insincere) + A + L (student))

DOWN
1 It's in course of getting marks and being well-informed (12)
UNDERGRADUATESHIP [UNDERGATSHIP] (UNDER (in course of) + GRADUATES (marks) + HIP (well-informed))
2 A water pipe may make agile run (7)
NARGILE[U] (anag of AGILE RUN)
3 Oilmen's kit's a cloth measure (6, two words)
DRILL PIPE [DRILPE] (DRILL (cloth) + PIPE (measure)) Adam seems to have painted himself into a corner with this hard-to-verify entry
4 The ultimate prophet's not one on trial (5)
MOSTEST [MOSTE] (AMOS (prophet) minus A (one) + TEST (trial))
5 Wheat fungus damaged truss (5)
RUST[S] [STURT] (anag of TRUSS)
6 Not being humourless, photograph's of gulls (8)
LAROID [RAILROAD] (POLAROID (photograph) minus PO (humourless))
7 Edentate has to live in a cave (6)
ANTBEAR [ANTBER] (ANTAR (cave) round BE (to live))
8 A group of flesh-eating bats seen in evening - note cat has wrong idea (12)
VESPERTILIONIDAE [VESPRTILONDA] (VESPER (evening) + TI (note) + LION (cat) + anag of IDEA)
10 Rattles around in tablet form (6)
STELAR[T] (anag of RATTLES)
11 A saucer-shaped depression - one found in a tailless yabby (4)
GILGAI [GILA] (GILGIE (yabby) minus last letter round A (one))
15 Pungently a bed of rock nearly found between the lines (8)
RACILY [RACIALLY] (RY (lines, railway) round A CILL (bed of rock))
19 Divine drink produces distension around horse joints (6)
SPA[E]VIN (SPAE (divine) + VIN (drink))
20 Garish, coloured and merry (7)
CRIANT [TANTRIC] (C (coloured) + RIANT (merry))
22 How German sees Czech spa town's not bad for misers (6)
CARLS [SACRAL] (CARLSBAD (Czech spa town) minus BAD) The German reference seems unnecessary and confusing as it is Carlsbad in English
24 European city - to the natives - offers confident advice, sensible but incomplete (6)
NAP[S]OLI (NAP (confident advice) + SOLID (sensible) minus last letter)
26 As much as base taken from cereal (4)
WHAT (WHEAT (cereal) minus E (logarithmic base))
27 One opposed to worker gaining independence? (5)
ANTI [TITAN] (ANT (worker) + I (independence))
28 Channel, not river, fish (5)
TENCH (TRENCH (channel) minus R (river))