BOBCAT kicks off the week…
A great puzzle with a feline Nina (as one expects). I struggled with some of the parsing, especially 13d, but still found it thoroughly enjoyable.
Thanks BOBCAT!

ACROSS
9. Naughty Seth and Marnie bathe together (2,3,4,6)
IN THE SAME BREATH
(SETH and MARNIE BATHE)* (*naughty)
10. Hottest spot gets lowest score? That’s unexpected (5)
HELLO
HELL (hottest spot) gets O (lowest score)
11. Already set up to adopt Times Modern (6-3)
LATTER-DAY
(ALREADY)* (*set up) to adopt T T (times)
12. Organ introit’s a revelation (3-6)
EYE-OPENER
EYE (organ) + OPENER (introit)
14. I agree to curb extremely abstract poet (5)
YEATS
YES (I agree) to curb A[bstrac]T (extremely)
16. Agent on commission transformed Runcorn bakeries (9,6)
INSURANCE BROKER
(RUNCORN BAKERIES)* (*transformed)
19. Total loss or guaranteed returns (not much)? (5)
GROSS
([lo]SS OR G[uaranteed] (not much))< (<returns)
21. Winger converts first of tries against Spain (9)
TURNSTONE
TURNS (converts) + T[ries] (first of) + ON (against) + E (Spain)
23. One may rumble an underdog, apparently (9)
SUBWOOFER
SUB (under) + WOOFER (dog)
25. Broadcast enchanted Cologne? (5)
SCENT
"sent" = SCENT (enchanted, "broadcast")
26. It could be that no pair volunteer to move (8,7)
RELATIVE PRONOUN
(NO PAIR VOLUNTEER)* (*to move)
DOWN
1. Beginning to feel the pinch? Mix the double gin with minimal tonic (10)
TIGHTENING
(THE GIN GIN (double) with T[onic] (minimal))* (*mix)
2. Prime Minister in general led by headless power unit (6)
ATTLEE
LEE (general) led by [w]ATT (power unit, headless)
3. At a discount, requiring less than 60% of capital (5,3)
BELOW PAR
BELOW (less than) + PAR[is] (capital, 60% of)
4. Bar bishop knocking out rector in protest (4)
BAIL
RAIL (protest, B (bishop) knocking out R (rector))
5. Dictator’s wise to constrain festival failing to exhibit a notice in the past (10)
YESTERYEAR
"wise" = Y Y ("dictators") to constrain E[a]STER (festival, failing to exhibit A) + EAR (notice)
6. Weird fringes of party supporting tribe (6)
CREEPY
P[art]Y (fringes of) supporting CREE (tribe)
7. Drivers’ club run by night probed by very nosy individual (8)
AARDVARK
AA (drivers' club) + R (run) by (DARK (night) probed by V (very))
8. Some people start to terrify with call for attention (4)
THEY
T[errify] (start to) with HEY (call for attention)
13. Routine sort of job in spinning setter’s keeping clear of (4-2-4)
NINE-TO-FIVE
Not really sure how this works… maybe something to do with the sun?
15. Mineral water in London park inadequately described (10)
SERPENTINE
Double definition (not quite sure what the "inadequately described" is)
17. Mushroom cocktail (8)
SNOWBALL
18. Dolores is to run rings around anyone standing up to her (8)
RESISTOR
[dolo]RES IS TO R[un] (rings around)
20. Polish assembly once cloaked in silence (6)
SMOOTH
MOOT (assembly, once) cloaked in SH (silence)
22. Manx cat the French officer raised as a wild animal (6)
OCELOT
(TO[m] + LE (the, French) + CO (officer))< (<raised)
23. Arrange play without regard to Pinter’s introduction (4)
SORT
S[p]ORT (play, without regard to P[inter] (introduction))
24. Ready to take two subjects at school (4)
RIPE
RI PE (two subjects at school)
13 is NI (IN spinning) + I’VE (setter’s) around (“keeping” ) NET OF
Thanks Bobcat and Teacow
I think SERPENTINE is “inadequately described” because it isn’t: it only has one bend.
Some very clever clues. Although I had all the crossers, I needed a word finder for nho TURNSTONE and SUBWOOFER.
Thanks Bobcat for the puzzle and Teacow for explaining it so well.
My last one in was 7d, after I had changed YATES to YEATS.
15D The title of the lake in Hyde Park is “The Serpentine”, so “Serpentine”, without more, only partly identifies it
I took 15d to be a triple definition: (Mineral) (water in London park) (inadequately described). The last definition relating to serpentine in the sense of difficult or overly complex.
Tricky but some great clues. Who would have known that insurance broker and Runcorn bakeries were anagrams? Love it.
Agree with your intro, Teacow – an enjoyable chewy puzzle!
I was thinking along the same lines as Simon S for “inadequately described” but I can’t say I’m wholly convinced by this or any of the other proffered explanations. Bagpuss’s suggestion seems most plausible (wise cat).