Neologification by Waterloo

Waterloo here offers one of his "sui generis" puzzles - similar to "OO Spectacles" in that there's a practical limit to the thematic words that could be used, and therefore we wouldn't expect to see the theme repeated. Here all (actually, most of) the entries were non-words coined by some kind of false etymology or derivation. Some of these were familiar, others whimsical, others quite difficult to pin down. The inability to check entries in Chambers made some of this puzzle quite tough, and Gregson had particular problems with ORNIFY (looked like it wanted to be ORNACY) and OPIALISE {many variants of OPIUM could have been candidates). Only the wordplay could yield the correct solution in each case.

So, an amusing theme, entertainingly composed, but probably not one to be repeated any time soon.

Across

1 Small case left singers outside open-mouthed, missing first and last quartets (12)
PORTMOUCHOIR [PORTMANTEAU]
(PORT + MO (OPEN-MOUTHED minus first 4 and last 4 letters) inside CHOIR)
10 Round and golden and full of precious metal (4)
ORED [False participle]
(O + RED (golden))
11 Saint leaving for fear, revealing citizens of old empire (5)
INCAE [False plural of INCA]
(IN CASE minus S(AINT))
12 Support tailless bird (3)
PIE
(PIER minus last letter)
14 Upset coming from fun starting (4)
UNST [UNSTABLE]
(Hidden in fUN STarting)
15 Mixing a martini, dropping in heavenly drink (6)
AMRITA
(Jumble A MARTINI minus IN)
16 Unworried thug returns endlessly (4)
TURB [Opp. PERTURB]
(BRUTE minus last letter, rev.)
18 Betray nothing for adherent (5)
SELLO [SELLOTAPE]
(SELL + O (nothing))
19 Associate of the Library Association keeping British books (4)
ALBA [False plural of ALBUM]
(B(ritish) inside ALA)
20 Bouquet not showing, for instance, refusal (5)
NOSAY [NAYSAY]
(NOSEGAY minus E.G.)
22 Elaborate condition in old-fashioned, slipping college (6)
ORNIFY [ORNATE]
(IF inside CORNY minus C(ollege))
25 Old fellow sinks gallons in farewell party (4)
FOGY
(G(allons) inside FOY)
26 If so, then god would be got a long way off (4)
DIST
(If "D is T" ...)
28 Flagging an exciting time on return (4)
ASAG [False nounal form]
(A + GAS rev.)
30 Encroach half-way, curled up, and fail to make out (4)
CERN [Opp. DISCERN]
(Jumble ENCR (half of ENCROACH))
31 Sele - to derive from flax? (6)
LINSEE [False present tense of LINSEED]
(SELE = "L in SEE")
33 In action, French Republic is well-nourished (6)
DERFED [Opp. UNDERFED]
(FR(ench) rev. inside DEED)
35 Lowbrow men with no limits, etc, all exult wildly (12)
EXTELLECTUAL [Opp. INTELLECTUAL]
(Jumble (m)E(n) + ETC ALL EXULT)
39 Run in seven agents causing suffering (4)
VIRI [False plural of VIRUS]
(R inside VII (Roman 7))
40 Harpo entangled someone given to pithy remarks (5)
APHOR [APHORISM]
(Anag. HARPO)
41 Last of Marx after mixing the apple business using seven screens (9)
HEPTAPLEX [DUPLEX, MULTIPLEX etc.]
(Jumble THE APPLE + X (last letter of MARX))

Down

1 Rashly spent on a back feeler (8)
POSTENNA [Opp. ANTENNA]
(Anag. SPENT ON A)
2 Spherical torus, topped and tailed twice, like an onion, cored (8)
ORBULOUS [False adjective]
((t)OR(US) + BULBOUS minus middle B)
3 Comes home lacking tea (no running water) and puts in the pot again (6)
REURNS [False verb form]
(RETURNS minus (TEA minus EA))
4 Deceive and smile wrily (5)
MISLE [False present tense of MISLED]
(Anag SMILE)
5 Aware of villain changing sides in basic principle (9)
ONTOLOGUE [ONTOLOGY]
(ONTO + ROGUE with L replacing R)
6 A party organiser gets into short trousers to make a home movie (7)
CAMCORD [False verb from CAMCORDER]
(A + MC inside CORD(uroys))
7 Woman from Mexico, perhaps, brave getting god in for nothing (8)
HERPANIC [Opp. HISPANIC]
(HEROIC with PAN replacing O)
8 Addict party losing head with pot initially, not cocaine (8)
OPIALISE [OPIUM]
(SOCIALISE minus first letter, P(ot) replacing C(ocaine))
9 Something a cowboy needs, relating to a bit of kip on the way up (5)
REATA
(RE + A + AT rev.)
13 Separate charm and ecstasy (3)
ITE [Opp. UNITE]
(IT + E(cstasy))
17 Policeman gathers you once had an affair (6)
PLOYED [Opp. DEPLOYED]
(YE inside PLOD)
21 Primitive country-woman - in France she follows you to begin with - right? (7)
YOKELLE [False fem. YOKEL]
(Y(ou) + OK + ELLE)
23 Mad queen leaves in that direction (4)
YOND
(YONDER minus ER)
24 Mature men attempt to follow "cool" (6)
FANTRY [Opp. INFANTRY]
(FAN + TRY)
27 Store up valuables for instance in box, perhaps (6)
TREASE [TREASURE]
(AS inside TREE)
29 To fall with one's horse - no centre to seat (4)
BECH [BECHER'S BROOK]
(BENCH minus middle letter)
31 Jean disguised in veil (4)
LEVI [False sing. LEVIS]
(Anag. VEIL)
32 African iris is a big hit before losing ends of sepals (4)
IXIA
(SIX + IS A minus two Ss)
34 Roll up! There's a lot of fun to be had on both sides (4)
FURL [Opp. UNFURL]
(FU(n) + R(ight) + L(eft))
36 Nimble bit of kleptomania (3)
EPT [Opp. INEPT]
(Hidden in klEPTomania)
37 Opt out - more would be excessive (3)
TOP
(Anag. OPT)
38 Somewhat less than moral fibre shown when going topless (3)
LAX
(FLAX minus first letter)