Times 25585 - Sting in the Head

I raced out of the starting blocks with this one but very soon began to realise that it was not going to be as easy as I originally thought. An hour later with much of the bottom half incomplete - mainly in the SW - I wondered if I would ever get through it. I never heard of 2dn, 15ac, 15dn or the archaism in 21ac. I knew the nymph at 18ac but simply couldn't bring her to mind and that blasted French pot thing at 14dn has caught me out before and immediately slipped from memory. I can't say I enjoyed this much, unlike yesterday's which took roughly the same time and was a pleasure to solve from start to finish.

* = anagram, dd=double definition

Across
1	PLAGUE - PRAGUE -R+L. I hope I am right in thinking that 'this' in the clue makes it clear which way round the substitution is supposed to work.
4	BOUFFANT - NAFF (common) reversed inside BOUT (attack)
10	PASTIME - The definition is 'activity' and it has the word 'past' in it. Beyond that is beyond me. On edit, please see the discussion re Pa's time below. I think joe's comment towards the end of the thread makes sense of it.
11	PRUSSIA - Hidden and reversed. The state ceased to exist in 1947.
12	TWIN - TWINe
13	BREAK SWEAT - With reference to beads of sweat.
15	SASSAFRAS - FAR* inside SAS, SAS. This tree is a source of oil used in medicine and perfumery.
16	ULTRA - dd, one with reference to ultra-sound treatment in medicine, dentistry etc.
18	OREAD - O (please), READ (look at), a mountain nymph that eluded me until the very end yet I had heard of her. My very first thought here was Heidi.
19	PAPERBACK - PAPER (Sun) is reversed in the clue. For those that may not know, NUS is the National Union of Students or of Seamen.
21	TRAIN WRECK - TRAIN (prepare), W (with), RECK (care). The last word is an archaism and this was indicated which was helpful.
23	LEAP - dd
26	MACHETE - ACHE (pain) + Torment, all inside ME
27	FARRAGO - FAR (long way), officeR, AGO (in the past)
28	PLACENTA - ChEw inside PLANT (vegetable, say), A
29	WEAKLY - Sounds like "weekly" (regularly)
Down
1	PIPIT - PI (good), PIT (hole)
2	ABSCISSAE - Anagram of CASES AS I Bow
3	UNIT - UNITe. I'm not sure if 'military' alone can define this, so 'team' would appear to be doing double duty. On edit, Mct points out that UNITy (one) is a better fit, leaving 'military team' available as the definition of 'unit'. Ta.
5	ORPHANS - (HAS PORN)*
6	FOUR-SQUARE
7	AISLE - Sounds like "I'll", apart from anything else.
8	TOAST RACK - TO (shut), A, S (small), TRACK (path)
9	BEARER
14	JARDINIERE - JAR (container), then I inside DINER (café), tablE
15	SHOOT-EM-UP** - Anagram of POSE MOUTH. A violent film or video-game apparently. **On edit, I mistyped this originally as SHOOT-ME-UP leading to the correspondence below.
17	TRADEMARK - ART reversed, DEnMARK. I'm not keen on 'are' clueing ART without an indication of it being old-fashioned, but apart from that 'are you' would surely clue 'art thou' rather than 'art' alone?
19	PERTEST - PER (for each), TEST (exam)
20	PACIFY - IF (condition) inside PACY (fast)
22	ACCRA - CC (small volume) + R (runs) all inside AA (Automobile Association - what drivers may need). It's the capital of Ghana.
24	PEONY - PE (exercise) ONlY (just)
25	ERIE - EyRIE (nest)