1 WASH UP debrief
Derived entirely from the wordplay, which is H(enry) inside WAS (used to be) and UP, at university, and an assumption that in some strange military or business world one might be a slang term for the other. Also assuming it had nothing to do with taking ones (or another's) trousers off. It turns out that it is indeed informal business speak for a sort of follow up or debriefing session (ODO) with much debate on whether it has a British or US origin. Maybe we can pin the blame on the Aussies.
5 NUMBERED Figured
I suppose in the sense of having numbers. N(ational) U(nion of) M(iners), half of BE(er) and RED for wine.
9 WEIGH OUT Jockey must do it at entry
Everyone else, of course, weighs in. Sounds like WAY OUT or exit.
10 OPENED Exposed
Writer is PEN, school CO-ED. Insert one into the other, take away the C(aught).
11 UNTRUE Not so.
Oh how we laughed! "You turn if you want to. The lady's not for turning". 10 October 1980, the newish PM (and not yet a Lady) refusing to change tack despite 3m unemployed. Apparently she didn't recognise the Christopher Fry literary reference either, but then she was only a chemist. You don't need to know any of that, though of course it smokescreens the clue. A contrived version of UTURN and (Maggi)E.
12 BRIOCHES Rolls
Recalling perhaps another earlier lady's response to the sufferings of the masses. Oomph is BRIO, and the rest is derived by chopping hire of CHEShire.
14 LITTER BASKET nothing much of value in it
Refuse=LITTER, and portfolio of investments=BASKET.
17 SAFE CRACKERS Those taking deposits illegally
Easy if you remember that, in crossword land a Peter is a SAFE. Not difficult to see that Dotty can be CRACKERS.
20 BRASSICA plant
Turnips and cabbages. Military officers are BRASS, here I(n) C(harge) of A
22 ASSISI Italian town
My nemesis-du-jour, reading the clue more cleverly than it was meant. I took the second I of Assisi and made it a T, just like the clue says, and ended up with ASSIST. Now if only I could explain why "no" means "assist". What you're meant to do is think of a word which means "second" (that'll be ASSIST, then), remove the T and add an I.
23 PODIUM to finish in the top three
Rather a hateful nerb (or is that voun?) common in Formula 1. Hate is ODIUM; put P(ressure) at the front.
25 SHE DEVIL Spiteful woman...
...who would of course become a good person if only she'd shed evil. Teehee!
26 DEMONIAC &lit
Take one of the M(iles) away from MANIC MODE, treat the letters diabolically, and you get COMEDIAN if you have no crossers and our true answer if you -um- don't.
27 SOLVER you
Setter points finger out of page. Only: SOLE, R(ight), and include V(erse). If you don't understand the clue, is it still true?
2 AGEING Elderly
A wise monarch might be a SAGE KING. Remove his first letters.
3 HIGH ROLLERS Those spending rashly
We have another school, this time a HIGH one, supported by waves which are ROLLERS. Can a high roller also be betting responsibly?
4 PROTECTOR Box at Lord's
A gentleman's genital carapace, usually in cricket, where a short pitched ball on middle might otherwise prove inconducive to continuation of the family line. PRO TEC TOR translates for private eye Hill.
5 NOTABLE Big noise
Or a person of importance, real or imagined. If you have NO TABLE, it's hard to play cards. Unless it's Snap. Or 52 Pick-em-up
6 MOOLI root vegetable
Hands up if you thought it was a food proceessor. Long and white it is, tasting vaguely of radish. Low: MOO, the L and I coming from the alternate letters of pLaIn.
7 EVE Lady.
Always: EVER. Since this is a down clue, pinching the bottom means removing the R. Stop sniggering at the back.
8 EYE LEVEL viewpoint.
Top quality is HIGH LEVEL. Speak it as you imagine Dick van Dyke would in conversation with Miry Poppins.
13 CASTS A SPELL Entrances
Lots of actors are CASTS, give them a turn or A SPELL.
15 BEE EATERS birds
If you take the F(ine) out of BEEFEATERS, you get these rather pretty birds. If you take the P out of Beefeaters, they'll lock you up in t' Bloody Tower and forget where they put Her Majesty's keys.
16 CAPRIOLE High jump
Most famously performed by the horses of the Spanish Riding School. Rebuild the letters of PLACE around Olympic City RIO.
18 CLASSIC of the highest quality
Our family's first car was a Ford Classic, which tended to bend in the middle, so perhaps not living up to this interpretation of its name. C(ricket) C(lub) takes into itself LASS (girl) and 1.
19 ASPIRE the opposite (of despair)
Good enough as an antonym once you realise you have to construct it from the letters of DESPAIR minus the first.
21 IAMBI feet
Poetical ones, that is. Today's hidden, in WillIAM BIg.
24 IBO African
From the Biafra (remember that?) region of Nigeria. Purists might suggest it should be Igbo, but we've only got three spaces. A BIO is a life story, drop the B one space.