I found this quite an easy puzzle, solving time 14 mins. I spotted a Nina (i.e. a hidden message) in the grid. In case solvers wish to look again first, I refer to this after the clue explanations.
* = anagram < = reversed
ACROSS
1 SUS SEX
4 STUBBS “Stubs”
10 HEALTH INSURANCE Cryptic definition I got straightaway which helped.
11 MAY OR
12 M U L(a)TITUDE
13 PARSE C
15 EXISTS Sexist with the first s moved to the end.
16 ST AN CE
18 BU(r)YING
21 I R R IT ATE D
22 LEHAR Rachel< less c (about)
23 THIN ON THE GROUND I think this refers to an ad from way back for muscle building (Charles Atlas). Definition = rare
24 NERVES (Severn)< with the er not reversed, I think
25 SHRE(W)D
DOWN
1 S (C HEM) A
2 S (EA(r)LY) HAM TERRIER r = king
3 (c)ENTER
5 T U R(i)GIDITY
6 BAN QUETING HOUSE (question huge)*
7 SW (ED) EN news<
8 FIL(l) MIC
9 IS OLDE(n)
14 SUNSTROK E (trunks so)*
17 E STATE
18 BAD GER(m)
19 MI(L)T ON
20 E (ROD) ED Dee<
22 LURCH Double definition
NINA: look in sixth and tenth rows
Please explain the wordplay for filmic. #8
Reduction in satisfaction over microphone used in cinema (6)
I assumed it was satisfaction = fill (as in “had one’s fill”), reduced (by one letter), on top of mic = abbreviation for microphone, giving filmic, meaning cinematic
How pleasant to have a straightforward puzzle from a fellow-setter that lasted for exactly the time the bus took to take me three miles in the pouring rain to the centre of Oxford!
Presumably all puzzles that you do, Quixote, are by a fellow-setter…