A single unresolved wordplay and what appears to be a mistake by our friend Azed. Difficulty about mid-range for Azed — not terribly easy but nonetheless solvable by mere mortals such as myself. See notes below for ALGEBRAic explanation — thanks to Andrew.
Across
1 OLD,M,OUST,ACHE
10 KARENNI – Thai tribe whose women stretch their necks with brass collars. Nope. Didn’t know this. But the comp. anag. doesn’t quite work — I think it’s supposed to be: (as neck-rings are)*=(Karenni’s grace)* but there’s a missing S on the right-hand side.
11 OLDY – hidden &lit.
14 AL(MAGES)T – treatise by Ptolemy. Alt=high-flown mood. Not sure why “as it were” required?
15 E,QUID – my last clue. Not sure why since it’s not particularly difficult given that quid is the second thing that comes to mind for pound (after L).
16 E(AS)E – AS is Roman coin.
17 MU[c]K,LU[c]K – Eskimo fur boot.
18 RIB,BON – it’s an “endless saw”. Tough clue since “endless” often tends to be a cryptic instruction. RIB=a framing timber.
19 BE(INN)ES,S – lots of misleading whisky on the surface here (Southern Comfort and Scotch) but none in the answer. Scots for comfortable. BEES are known for their work ethic: “busy as a bee”.
23 ARCH,A,IS,T[rendyism]
27 [tai]L,OWRIE – two unknowns needed to solve this equation: LOWRIE=Scots nickname for fox and owrie=Scots drooping.
28 A,ST(R)AY
30 IWIS = two meanings: Maori tribes indeed and certainly (derived from German gewiss).
32 S[ti]WEEP – remove it from rev(peewits=birds)
33 S(AT)IATED – AT in (a diet’s)*
34 G,O(N)K – slang prostitute’s client.
35 [p]EARLIES – pearlies are costermongers and EARLIES are (early) potatoes.
36 H(EDGE,MUST)ARD – roadside weed
Down
2 LASQUE – another hard clue. Replace ALI in Lalique http://en.wikipedia.org/wiki/Ren%C3%A9_Lalique (French jewellery designer) with AS to get LASQUE which is an Indian diamond.
3 DROUKIT – sounds like “drew kit” (something a new recruit will have done early on). And is Scots “drenched”. I initially guessed DROOKIT which made 15A (EQUID) a lot harder of course.
4 MERIL – hidden – counter used in board game called… merils.
5 [d]U(NLIK*)E[t]
6 SI(MA)RS – cymar which is an undergarment.
7 T(O)AD
8 ALGEBRA – not sure about the wordplay: “School subject: rejoice endlessly when it’s given up before academy?” – academy is A or RA. The rest?
9 H,YSSOP* – holy-water sprinkler
12 STENO*,TYPE
13 C(EMABL*)IST – cist=tomb. A cembalo is a harpsichord.
20 NUR,SING – rev(run=roll).
21 S(IR)ENIA – I[slande]R in (in sea)*. Class of manatees. Found under sirenian in Chambers.
22 COW,AGE – neat=cow is a familiar crossword idiom.
24 RESTEM – hidden. Shak. “force back against current”.
25 C(A,DEA)U – DEA = Latin Goddess.
26 S(A)ETER – A in trees*. Animal shelter.
29 T(W)ILT – quilt=puff in dialect.
31 WAKE – “a track on land” (rare). Two meanings.
Thanks Ilan – 8d is BE GLA[D] rev. + RA
Re your query at 14a. Alt. is ‘altitude’, which means ‘height’. As the use of this word in the clue wouldn’t make sense, we have “high-flown mood”, an extended, poetic, fanciful description – suggested by the qualifier “as it were”.