Quite a tricky challenge from Monk this morning.
A most enjoyable puzzle, with a lovely nina. Some tricky words and parsing, but all completely fair of course. Many thanks to Monk! And happy hoaxing to all on this day.

ACROSS
6. Reckless behaviour of handler is so upsetting (13)
FOOLHARDINESS
(OF HANDLER IS SO)* (*upsetting)
10. Each rounded turbine blade catches minute component of Spitfire? (9)
AEROMOTOR
EA< (each, <rounded) + ROTOR (turbine blade) catches MO (minute)
11. Business plugs revolutionary top-notch clubbing Mecca (5)
IBIZA
BIZ (business) plugs AI< (top-notch, <revolutionary)
12. For and against hosting extremely chatty star (7)
PROCYON
(PRO (for) and ON (against)) hosting C[hatt]Y (extremely)
13. Staff centre on standby initially absorbing pressure after unknown number sacked (7)
SCEPTRE
CE[n]TRE on S[tandby] (initially); absorbing P (pressure) after N (unknown number) sacked
14. Struggling, ultimately for years, to get best value above all else? (3,5,5,2)
SET GREAT STORE BY
([fo]R (ultimately) + YEARS TO GET BEST)* (*struggling)
18. Off and on, groups betray philosopher (7)
RUSSELL
[g]R[o]U[p]S (off and on) + SELL (betray)
21. “Un-American” newspaper item about European network (7)
RETICLE
[a]RTICLE (newspaper item, un-American, i.e. no A) about E (European)
23. Quietly follow discussion on society (5)
STALK
TALK (discussion) on S (society)
24. Specialist food store with cheerful drug causing frenzy (9)
DELIRIANT
DELI (specialist food store) with RIANT (cheerful)
25. Act shrewdly? Old boy fobs one off (2,7,4)
BE NOBODY’S FOOL
(OLD BOY FOBS ONE)* (*off)
DOWN
1. Cross over job part of the way in? (8)
DOORPOST
ROOD< (cross, <over) + POST (job)
2. Sadly, mucilage covering left and central parts of false eye (6)
GLUMLY
(GUM (mucilage) covering L (left)) and [fa]L[se] [e]Y[e] (central parts of)
3. Decorates bar up in Swansea after walls repeatedly stripped (6)
ADORNS
ROD< (bar, <up) in [sw]ANS[ea] (after walls repeatedly stripped)
The ‘walls’ of the word ‘Swansea’ are the outer letters, and we remove (strip) these repeatedly, so more than once
4. Some rampant bore hit naïve, ignoble protagonist (8)
ANTIHERO
[b]ORE HIT NA[ive]< (some, <rampant)
5. Help animal point away from front (6)
ASSIST
Cryptic definition
“ASS 1st”
7. Receiver’s advance payment not available after noon (7)
ANTENNA
ANTE (advance payment) + (NA (not available) after N (noon))
8. In the morning, sap rising in soft split on tall plant (6,5)
PAMPAS GRASS
(AM (in the morning) + SAP< (<rising)) in P (soft) + GRASS (split)
9. Peter and Betty swimming round lake with small means of protection? (6,5)
SAFETY BELTS
SAFE (peter) and (BETTY* (*swimming) around L (lake)) with S (small)
15. Bond running free, overturning defeat (4,4)
REEF KNOT
FREE* (*running) + TONK< (defeat, <overturning)
16. Keenly, exhausted hiker fills potty (7)
SHRILLY
H[ike]R (exhausted) fills SILLY (potty)
17. Vegetable that’s key with almost all spicy wraps (8)
ESCHALOT
ESC (key) with (AL[l] (almost); HOT (spicy) wraps)
19. Disturbed Poles catching fish (6)
SHAKEN
S, N (poles, South, North) catching HAKE (fish)
20. Left upland country, rising around university town in Shropshire (6)
LUDLOW
(L (left) + WOLD< (upland country, <rising)) around U (university)
22. Frugality essentially buried in most of the Financial Times (6)
THRIFT
[bu]RI[ed] (essentially) in (most the TH[e] + FT (Financial Times))
New words for me today: RIANT, TONK, and ESCHALOT. All good, thank you Monk and Oriel
10a. I think this is AE (each rounded) ROTOR (turbine [blade?]) catches MO.
19d. I had SKATES (disturbed STAKES), but SHAKEN is better, agreed.
Thanks both!
Thanks Olof @2 – yes, I had not accounted for the E in 10a. I’ll amend this. Thank you!
Thanks Monk and Oriel
5dn: I had ASS (animal) + [g]IST (point away from front)
I knew I’d missed something
Thanks all and Paul
I did not follow the blog for 5d. Pelham Barton’s alternative@4 makes sense but it does seem a very tortuous way to remove the first letter of gist. Does anyone have another idea?
A very necessary blog for a tricky puzzle which left me with several clues I hadn’t got to the bottom of. And I missed the rather good Nina. Thanks both.
Tricky indeed
FOOLHARDINESS is such a great anagram and surface. I also liked DOORPOST
There were so many NHOs that it started feeling silly – keeping with the theme, I guess
Thanks Monk and Oriel
What a great puzzle! I parsed 5d as PB.
Thanks for the “gist” suggestion which is torturous but better than anything I came up with (“1st” just doesn’t use “point away from” so can’t be right). A clever nina down the diagonal and plenty of sound clues, though I was not sold on “rotor” for “turbine blade” – the rotor is the hub or the whole blade assembly (i.e. multiple blades). A blade is, well, a blade. But then I had an unparsed “altimeter” for a while which may have put me off the clue!
“Doorpost” =”part of the way in”. Very droll! Thanks Monk and Oriel.
Thanks Monk. I found this on the difficult end of Monk’s spectrum and I eventually revealed a couple of stumpers. I can’t complain, however, because all was fair. I liked RUSSELL, RETICLE, ANTIHERO, SHAKER, and THRIFT. I looked for a Nina but failed to spot Fool on the Hill, perfect for today. Thanks Oriel for a much needed blog.
10ac: Chambers 2016 p 1355 gives rotor n a rotating part, esp of a dynamo, motor or turbine. It seems to me that a turbine blade is a part of a turbine which rotates. That is good enough for me.
Quite hard, with some unfamiliar words. I could not parse ASSIST, but noted [g]ist???, as suggested above. That still does not seem quite right, i.e., the clue should say “front away from point,” right?
5dn revisited, further to various comments above, I read it this way: Imagine the word GIST being formed in a space outside the grid, perhaps the jotter pad that is provided on the pdf. Leave the G where it is, and take the rest of the word away from the G to place it in the grid.
I’m sorry, I still don’t understand this “gist” business, but let it pass. I’m also confused about the repeated stripping of walls in 3d. There may be two walls to strip but surely that’s not stripping them repeatedly? In 4d, is “rampant” supposed to indicate “backwards? If so it’s a very odd way of expressing it. Perhaps it is simply indicating an anagram, which a reversal of letters is, of course.
I struggled today (as you will see). It was one of those days when I had to be firm and stop trying because I spend too much of my life doing crosswords!
Further to my comment above, I think I see why I’m confused about the “walls”. I took Sw and ea to be two walls, not four.
4dn: Chambers 2016 gives “rearing” as the first definition of rampant (p 1290) and then rear² “to raise, cause or help to rise; to set up or upright” (p 1299), so I am happy with “rampant” as a reversal indicator in a down clue.
I got PAMPAS GRASS, but how does GRASS = split?
Adders @19: Grass is slang for ‘inform on’; split is slang for ‘divulge secrets’; I believe this is the connection.