This was a fairly typical Monday offering with enough about it to give pause to all but the most expert solver. I managed it in 45 minutes with one wrong and a bit of a guess at my last in, wondering how a lion might be an idol. Or maybe I got that wrong too. We shall see...
1 OPTIMISE - OP + IM in SITE* - a nice one to get us started.
5 H + OB+N+OB - 'after hours' was the giveaway here.
9 SAD + D+LE+RY - I was racing round the Isle of Man when I should have been casting my mind back to recall what it was like to exercise temperance.
10 TATTOO - 'looking back' is the reversal indicator and 'over' is the cricketing abbreviation O, so O + O (old) + TT (times) + AT reversed.
12 FIBRILLATION - I rather liked this, and the word itself tickles my fancy in a Ken Doddish kind of way; FIB + ILL in RATION.
15 Would it make you cry if we cut this?
16 SWAZILAND - anagram of LIZA[R]D and SWAN for the HIV-ravaged country with the lowest life expectancy in the world.
18 SNOWDONIA - NOW + DON in S[k]I[h]A[t] - 'now' for fashionable may be downright ugly but it's in the dictionary.
19 CHIEF - no excuse at all for writing 'thief', as the wordplay - and word order - makes it crystal clear that the felon not only loses his head but gets crowned with a 'c' for good measure, 'van' doing its opposite of rearguard thing.
20 SQUASH (press) LADDER (run in tights) - I hated these things (squash ladders not tights) as I always seemed to be near the bottom.
24 I'll follow the setter and make this an omission clue.
25 DELEGATE - [syndicat]E + LEG + in DATE - a little lifting and separating is all you require to attend this event.
26 EA(S)TER - if it's not a cooker then it's an eater, though both are in short supply in Blighty after the poor insects couldn't scramble in all that rain.
27 SKITTISH - many will have just bunged this in, but for the record it's SH for mum (as in 'mum's the word') enclosing KIT (clothing) and a reversed verbal model (TIS).
1 OUSE - there must be dozens of River Ouses in England; one of them is supplying today's non-dodgy homophone.
2 TI(D)E - thanks to dictionary.com for coming up with the 'turning point' definition (AKA 'a critical point in time'), even if it doesn't quite convince me, when all I could think of was the 'alternate rising and falling of the sea' meaning.
3 MULLIONED - LION in MULE + D[iamonds] - a number of factors contributed to this being my last in, not least the fact the word itself was so unfamiliar as to be functionally unknown. Anywyay, in crossword land, when a mule isn't being a hybrid it's being, as here, a woman's slipper, and when diamonds aren't being ice they are being a minimalistic Mephistoesque D. Then there's the lion. The best I can come up with for him, after extensive Googling, is the feline that appears to be worshipped along with his mount, the goddess Amba, in Hinduism. Oh, and if a window or screen is mullioned, it contains a vertical division.
4 SURPRISINGLY - another that many will bung in from the checkers. For the record, it's P + RISING in SURLY.
6 The down omission
7 NITRIC ACID - IC in TRAIN* + CID - I sometimes wonder why God gave us all this excess Nitrogen if he knew what we were going to make of it.
8 BROWNED OFF - another phrase I like for some reason, probably because I feel like this quite a lot of the time. It's BROW + NE + DOFF.
11 CLEAN AND JERK - yet another phrase that rather tickles my fancy, and thus gets my COD, even if it's sister event in the weightlfiting discipline has an even raunchier name. One of my favourite moments in the Olympics was when the North Korean pocket battleship of a female weightlifter, weighing 100 pounds, made it onto the podium at her final attempt behind a Chinese and a Japanese who looked, for some reason, far better nourished. It's an anagram of DANCER and ANKLE containing J[udge].
13 POSSESSIVE - 'controlling' as in helicopter parents. With a daughter just starting at boarding school in England, that would make my wife and I Skypelicopter parents. This time we have POSSE as the company for a change chasing [ad]VISES*.
14 BINOCULARS - was it only me who thought of the dog called Colin in Baldrick's rotten borough being interviewed by Vincent Hanna? Probably. Glasses is the literal, and BARS around COLIN + U* the wordplay.
17 INCLEMENT - our second, I think, letter substitution clue, where R becomes L.
21 I'm going scratch this one too.
22 TAX+I - like the muggins that I am, I put Bali and only got the real answer when I stopped to work out the wordplay and saw that tax meant try.
23 Hidden omission.