Thank you to Yank. Definitions are underlined in the clues.
Across
1. Iron Man is a woman (6)
FEMALE : FE(symbol for the chemical element, iron) + MALE(a man).
4. Criticise character from Athens, thus creating sort of poetry (3,5)
RAP MUSIC : RAP(to criticise/pan) + MU(twelfth letter/character of the alphabet of the language of Athens, Greece) + SIC(Latin for “thus”/so, to indicate that something has been quoted as originally written).
Defn: … since it shouldn’t be called music.
9. Snack: portion of spinach ossobuco (6)
NACHOS : Hidden in(portion of) “spinach ossobuco”.
10. Photo, a character from long ago: outspoken rogue (8)
PICAROON : PIC(short for “picture”/a photo) + A + homophone of(… outspoken) “rune”(a character from an ancient/long ago alphabetic script).
Defn: …/scoundrel.
11. Engaged in drink, CIA annulled cryptic system of old Rome (6,8)
JULIAN CALENDAR : JAR(slang for a glass of beer or alcoholic drink) containing(Engaged in …, …) anagram of(… cryptic) CIA ANNULLED.
13. Plunderers’ music fest in part of Winchester? (5,5)
RIFLE STOCK : RIFLESTOCK(what you might call a music festival of plunderers/riflers, after Woodstock, the 60s music festival, a la “gate/Watergate” – I think). And I assume that the “rs” in “riflers” is dropped when forming the compound work.
Defn: …, a type of rifle.
14. Prey for an aardvark: one Contra (4)
ANTI : ANT(prey for an aardvark, an African mammal that feeds on ants, and termites) + I(Roman numeral for “one”).
Defn: …/opposite.
16. God backflips into canal (4)
SUEZ : Reversal of(… backflips) ZEUS(supreme god in Greek mythology).
18. Lifted off around two, say: it may require a shot (5,5)
FIELD EVENT : Anagram of(… off) LIFTED containing(around) EVEN(describing a number divisible by two, an example of which/say, is two, of course).
Answer: A … in athletics, an example of which/may is the shot put, using a heavy metal ball, a shot.

21. Get ogress irritated and annoyed with symbol (2,7,5)
ST GEORGE’S CROSS : Anagram of(… irritated) GET OGRESS plus(and) CROSS(annoyed/angered).

23. Smooth talker on X, say (8)
OPERATOR : Double defn: 1st: …/persuasive person; and 2nd: In mathematics, a symbol signifying an action to be done with specific values, of which X, the multiplier symbol, is an example/say.
24. Arrest in general location reversed (6)
COLLAR : Hidden in(in) reversal of(… reversed) “general location”.
Defn: …/capture.
25. Sob stories in Slate? (3,5)
SAD TALES : Reverse clue: Anagram of(SAD) TALES = SLATE.
26. Turkish bigwig covers arse, it may be hidden (6)
AGENDA : AGA(a title for a Turkish bigwig/respected man) containing(covers) END(one’s bottom/arse).
Down
1. Satisfactory punishment (4)
FINE : Double defn.
2. Ingredient of comfort food covering rear end of Shakespearean (7)
MACDUFF : MAC(short for “macaroni”, an ingredient of Mac and Cheese, comfort food) placed above(covering, in a down clue) DUFF(one’s buttocks/rear end).
Defn: A … character, specifically from the play, Macbeth.
3. Celebrated rugby player offside (8)
LIONISED : LION(a member of rugby union team, the British and Irish Lions) + anagram of(off) SIDE.
5. Amoebae and such travelled nautical miles, giving up sex in retrospect (11)
ANIMALCULES : Anagram of(travelled) [ “nautical miles” minus(giving up) reversal of(… in retrospect) “it”(euphemism for “sexual intercourse)” ].
Defn: An archaic term for microscopic organisms, ….
6. Lower, but greater (6)
MEANER : Double defn: 1st: …/more despicable; and 2nd: …/more excellent, as in “his talent is … than hers”
7. Originally, source (excessively loud noise) was a sub (5,2)
STOOD IN : 1st letter of(Originally) “source” + TOO(more than required/excessively) + DIN(loud noise/a racket).
Defn: …/substitute.
8. Hustler’s damage includes train wreck (3,6)
CON ARTIST : COST(expense/damage, as in “what’s the damage for this lunch?”) containing(includes) anagram of(… wreck) TRAIN.
12. Singer wanting cabbage to whip up piece in advance (3,4,4)
NAT KING COLE : COLE(term for the vegetable “cabbage” and the like) placed below(… in advance, in a down clue) [ reversal of(… up, in a down clue) TAN(to whip/beat severely) + KING(a piece in chess) ].
13. Don’t play at stalls where Yanks relieve themselves? (4,5)
REST STOPS : REST(to stop doing, eg. “don’t play”) plus(at) STOPS(stalls/ceases operation, such as with a motor vehicle).
15. Drunken peer falls into brewski-chugging contraption in drinkers’ diversion (4,4)
BEER PONG : Anagram of(Drunken) PEER contained in(falls into) BONG(a contraption, in this case a water pipe for smoking marijuana – I’m not sure of the connection with “brewski-chugging”/drinking beer rapidly).
Defn: …, a drinking game.

17. E, for Einstein? (7)
EGGHEAD : Reverse clue: 1st letter of(…HEAD) EGG = E.
Defn: An example of which/? is ….
19. € is based on this Greek character harboured by lone lunatic (7)
EPSILON : PSI(a character/letter in the Greek alphabet) contained in(harboured by) anagram of(… lunatic) LONE.
Defn: €, symbol for the euro currency is based on this Greek letter.
Answer: ε written in lower case.
20. 75% of plátanos turned into something romantic (6)
SONATA : Reversal of(… turned) 6 out of 8 letters of(75% of) “plátanos”.
22. Killer imprisoned in Windsor Castle (4)
ORCA : Hidden in(imprisoned in) “Windsor Castle”.

A fine finish to the week, this was a pleasant late night solve. I smiled at the image conjured up by SUEZ and also enjoyed RAP MUSIC, RIFLE STOCK, ST GEORGES CROSS, BEER PONG, REST STOPS and EGGHEAD. Was pleased to finish with the nho ANIMALCULES by working out the anagram fodder and having all the necessary crossers.
OPERATOR reminded me of this ear worm https://youtu.be/4TYv2PhG89A?si=0UPuFORXaE59qTlP
Ta Yank & scchua.
Comment #2
Thanks Yank and scchua
I’m adding Yank to my “don’t bother with” list. Too many US usages that were totally unfamilar to me. Take 13d, for example – I had heard of REST ROOMS, but not “stops”. DUFF for “rear end”? BEER PONG?
For 6D MEANER, I assumed that the second def referred to the informal use of ‘mean’ in expressions like ‘pack a mean punch’ or ‘make a mean scrambled eggs’. Wouldn’t expect to hear this in the comparative, though.
My only nitpicks here were ‘on’ as a connector in 23A (my LOI), and ‘brewski-chugging’ in the context of the bong. Enjoyed the rest immensely, particularly RIFLE STOCK.
I’ve enjoyed learning some new terms this morning. Useful additions to my vocabulary. Also, did not know the X meaning, scchua, thank you. I was thrown by entering REST ROOMS but I shouldn’t have done that as I was very unhappy with “rooms” as a synonym for “stalls.” Broke my own rule there: if you cannot parse it do NOT bung it in. Well done, Yank, on luring us into thinking this was too easy peasy and then clobbering us with unexpected but fair formulations.
Both Beer Pong and brewski-chugging unknown to me, but a device for the latter does seem to exist….
https://en.wikipedia.org/wiki/Beer_bong
Great puzzle, lovely misdirections in places eg 25ac sad tales and 3d offside. Thanks to Yank and scchua
RIFLE STOCK my favourite, probably due to the time taken to get it! Patted myself on the back for knowing ANIMALCULES, despite trying to shoehorn in a version of unicellular for some time. Came here to figure out MEANER.
I can begin to have some sympathy with the French and their latest assault on English terms in everyday use (Black Friday being the most recent). Whatever happened to the Macaroni Cheese I grew up with? Now the US Mac and Cheese seems to have taken over…
Thanks Yank and scchua
Doofs @7
It’s usually the even worse “mac ‘n’ cheese”.
A slow start but once I got the 13s, it opened things up. Liked 23A OPERATOR for the maths link. Thanks to scchua and Yank.
Favourite: EPSILON because of ELON being anagram of ‘lone’ (lunatic).
New for me: BEER PONG; DUFF = rear end.
Was not keen on RIFLE STOCK as Woodstock was all about peace and love, whereas rifles are the opposite or at least about violence and killing.
I could not parse 12d and the mathematical bit of 23ac.
A number of things, not all of them American, that I didn’t know this morning: the beer BONG (thanks Emcsquared@6), OPERATOR=smooth talker, MEANER=more excellent, COLE=cabbage (couldn’t parse NAT KING COLE at all, though I guessed him from the enumeration), DUFF=rear – and I wouldn’t ever have thought of MAC’n’cheese=comfort food as I don’t like the stuff at all. So a rather unsatisfactory result. But I enjoyed many others, especially ANIMALCULES, JULIAN CALENDAR, PICAROON, RAP MUSIC. RIFLESTOCK made me smile. Thanks Yank, and scchua for the nicely illustrated blog.
Glittering prizes for EGGHEAD, LIONISED and the SMOOTH operator for the X misdirection – X also a place where comment #2 might be more at home
I also wondered about MEANER but figure “she plays a mean fiddle” was the required sense and Chambers has Skilful or Excellent (slang)
I thought there were very few Americanisms overall and we even got something to keep the flag-shaggers happy
Cheers S&Y
IMHO if an archaic term is used it should be indicated, particularly an esoteric one.
‘Amoebae and such once travelled nautical miles, giving up sex in retrospect’ would do it. 🙂
Thanks to Yank and scchua
I agree with Muffin#3 and #8. Not much joy for me with this setter.
Should have twigged the misdirection in RIFLE STOCK much sooner as I [re]watched Shaun of the Dead earlier this week. Recommended! Spoiler: the Winchester in the Winchester is loaded.