Solving time: 29 mins I spotted a theme, I think, mentioned under the relevant clues.
< = reversed * = anagram
ACROSS
1 DUN MOW FLIT CH This is the theme, I think. I was very pleased to work out the answer (which is completely new to me) entirely from the straightforward wordplay and then try to discover what is what all about. Apparently a flitch (bacon) is given to couples who satisfy a jury in Dunmow (Essex) about their relationship. Goes back to medieval times (when it was very well known seemingly) and has been revived in recent times. The symmetrically opposite 27 across is also relevant, I think, as noted below.
10 A VA (I) L (e) One of two references to e = ecstasy in this puzzle, defined here as drug, in 25 as tablet
11 RAD I OGRAM Margo< Rad = excellent (from US apparently).
12 KOHL “Coal” Cosmetic used as eye make-up
15 HYDROZOA (hard oozy)*
19 TUR GID (Dig rut)<
20 ALASTAIR a last air (swansong)
25 SAUTERN ES (Nature’s)* + es ‘bubbly’ indicates the anagram
27 C HE ESEMONGER (some green)* ‘crumbling’ a very apposite anagram indicator. I solved this first time round and the word struck me as unusual (but it does exist). I realised why later because a particular cheese monger famously sought 1 across, I discovered. So I think this may be a theme item also.
DOWN
2 UGLY First and last letters.
3 MARAT H ON
5 LEOPARDESS (Eros lapsed)* Another word I looked up to confirm it existed, it’s in my Concise OED.
8 BACK CHA(r)T
9 CATHODE-RAY TUBE Box = TV
17 S (ERE NI) TY NI = Northern Ireland = province
21 MAG NUM(b)
19a – I think a clever double def,as in order to dig one’s rut a degree of turgidity would be preferable?
Was reasonably happy after verifying in Concise OED that this was as explained – a single definition and the two wordplay parts reversed.
I have to admit not knowing a cheesemonger sought the flitch (with the aim of making savoury snacks, I assume), so this was unintentional.
Not everything is a Nina…
Sorry that wasn’t a query of the answer, just an aside on the definition of turgid as swollen rather than pompous.
Having missed Phi’s Nina last week, I looked for too much this time! Thanks to Phi for clarifying it. Re Jos’ point, I never knew till today that turgid meant pompous but it does (Concise OED confirms) – I always thought turgid meant just what Jos says it does, with particular reference to boring writing.