Thank you to Jason. Definitions are underlined in the clues.
Across
8. New green energy is default (6)
RENEGE : Anagram of(New) [ GREEN + E(symbol for “energy” in physics ].
Defn: To go back on a promise or contract/…, as with a loan.
9. A time in rudely playing around? (8)
ADULTERY : A + [ T(abbrev. for “time”) contained in(in) anagram of(… playing) RUDELY ].
Defn: … sexually with someone who’s not one’s spouse.
10. Push back bowl over bananas (4)
NUTS : Reversal of(Push back) STUN(to bowl over/astound).
Defn: …/insane.
11. Person from the defence runs down missing leader (10)
RESPONDENT : R(abbrev. for “runs” in cricket score) + “despondent””(in low spirits/down) minus its 1st letter(missing leader).
Defn: A person who has to defend against a petition filed in court.
12. Pen opening to comedy with maturity (4)
CAGE : 1st letter of(opening to) “comedy” plus(with) AGE(state of being old/maturity, as in “wisdom comes with age”).
Defn: …/an enclosure, in this case with bars.
13. One in headphones in front of inferior group of stars (5,5)
CANIS MINOR : I(Roman numeral for “one”) contained in(in) CANS(informal term for “headphones”) plus(in front of) MINOR(of lesser importance/inferior).
Defn: A small constellation/… in the northern sky.
17. Top section adopted by old soldier (4)
VEST : S(abbrev. for “section”) contained in(adopted by) VET(short for “veteran”, a former/old soldier).
Defn: …/a garment for the upper body.
18. Playing with bounder’s contract (5)
INCUR : IN(taking part in an activity/playing) plus(with) CUR(a despicable man/bounder).
Defn: …/to catch and suffer from, eg. a disease.
19. Unhappy about husband’s ostentation (4)
DASH : Reversal of(… about) SAD(unhappy) + H(abbrev. for “husband”).
21. Doctor ran testing for cosmetic lotion (10)
ASTRINGENT : Anagram of(Doctor) RAN TESTING.
23. One performs in the deep lake (4)
SEAL : SEA(the deep/the ocean) + L(abbrev. for “lake”).
Cryptic defn: An example of a trained animal performing tricks, and whose natural habitat could be a deep lake.
24. Eg, Albert to survey mountain range (5,5)
WATCH CHAIN : WATCH(to survey/to observe) + CHAIN(a connected series of mountains/a mountain range).
Defn: An example is one named after Prince Albert.
A double one with a pocket watch at one end and maybe a pocket knife at the other:

28. Free eastern trip (4)
RIDE : RID(to free/to remove something unwanted) + E(abbrev. for “Eastern”).
Defn: … on a horse or in a car, say.
29. So enter a new-found boom (8)
RESONATE : Anagram of(… new-found) SO ENTER A.
30. Among French you will be involved in certain stitch up (6)
SUTURE : TU(“you” as spoken among the French) contained in(will be involved in) SURE(certain/definite).
Defn: To … thus:

Down
1. Report includes good tip from umpire for control (8)
REGULATE : RELATE(to report/to give an account of, say, an event) containing(includes) [ G(abbrev. for “good”) + 1st letter of(tip from) “umpire” ].
2. Recently appointed landlord holding second bulletin (10)
NEWSLETTER : [ NEW(recently appointed, as in “the new commissioner”) + LETTER(a landlord leasing out his property) ] containing(holding) S(abbrev. for “second”).
3. Certain rot put otherwise in withdrawal (10)
RETRACTION : Anagram of(… put otherwise) CERTAIN ROT.
4. Checks for mothers (4)
DAMS : Double defn: 1st: Holds back; and 2nd: Female parents of animals.
5. Possibly you’ll get about with bachelor in it? (4)
AUTO : Reverse clue: Anagram of(Possibly) AUTO containing(with … in it) B(abbrev. for “bachelor”) = (you’ll get) “about”.
Answer: Americanism for the “it” in ….
6. What improves grip of stable which grows? (4)
STUD : Double defn: 1st: … of footwear with the ground; and 2. Descriptive of a farm/stable in which animals are bred.
7. Public Relations in the process of dealing with prompt (6)
PRONTO : PR(abbrev. for “public relations”) + [ON TO](in the process of dealing with/getting to deal with something, as in “I’ll be getting on to that subject in a moment”).
14. Comfortable place in hotel in city on the Riviera (5)
NICHE : H(letter represented by “hotel” in the phonetic alphabet) contained in(in) NICE(city on the French Riviera).
15. Look carefully, cruise isn’t off (10)
SCRUTINISE : Anagram of(… off) CRUISE ISN’T.
16. Not divided up though foolhardy, as they say (10)
INDISCRETE : Homophone of(…, as they say) “indiscreet”(foolhardy/careless).
20. Ideal flag (8)
STANDARD : Double defn: 1st: Level of quality, the highest of which is an …; and 2nd: A military or ceremonial ….
22. Sun hell? ____ may be what you want (6)
SHADES : S(abbrev. for “sun”) + HADES(hell/the underworld).
Defn: What you may want when the sun is beating down as hot as hell on you.
25. Hack PC, hard to get in (4)
CHOP : COP(slang for “policeman”, an example of which is a “police constable”, abbreviated to “PC”) containing(… to get in) H(abbrev. for “hard”).
Defn: …/cut with heavy blows with, say, an axe.
26. Family group left in pokey (4)
CLAN : L(abbrev. for “left”) contained in(in) CAN(like “pokey”, slang for prison).
27. Part of sequoia certainly hides another tree (4)
ACER : Hidden in(Part of …) “sequoia certainly”.

I was defeated by 5dn, and still don’t understand the answer given by our blogger although I do now see the wordplay. Why is the Defn: Americanism for the “it” in ….
Otherwise a nice puzzle. Thanks setter and blogger.
Steven @1 – possibly you’ll get about in a car/AUTO? AUTO is the American version of the English car – so it in that sentence. It was my last one in too, looking in bemusement at _U_O.
Enjoyable puzzle, thank you to Jason and scchua
Thanks Shane. That makes sense. I was overthinking this as possibly something to do with IT computing or sexual congress….a misspent youth perhaps!
For the record, Americans say “car” far more often than they say AUTO. I think the only context in which the latter comes naturally is in the phrase “auto insurance”, which everyone who owns a car has.
Thanks Jason and scchua
5dn further to earlier comments: auto is marked as US and Canadian informal in Collins 2023 p 133, and as chiefly N Am in Chambers 2016 p 99. There is in any case nothing in the clue that requires the answer to be an Americanism.
An unusual grid layout, and potentially a nasty one – all the perimeter squares unchecked, with twelve 4-letter solutions, and eight 10-letters in the mix.
However, a kindly and “direct” set of clues helped the solver, and it was enjoyable to complete.
I liked the “bachelor” device, in 5(d), AUTO; also, the use of “playing around” in 9(ac), ADULTERY.
Funny word, “AUTO”, and the format [~U~O ] stumped me for a while; it’s from the Greek αυτο(ς), meaning roughly, “one’s self”, though in Greek it’s pronounced “afto”.
Automobile, is “αυτο – kineto”, so: it “makes oneself moblle”.
ALBERT 24(ac), is a nice DBE, though may be one for us oldies? Coined in Victorian days, Prince Albert sported the watch chain, hence the moniker. I’ve alway thought of it as a “London/Cockney” expression.
Nice stuff throughout, from Jason; cheers, and to scchua.
Thanks for the blog , very good set of clues , lots of variety .
I had ADULTERY as an &Lit otherwise the anagram of rudely is not indicated .
AUTO is a very clever clue , a sort of compound anagram with the whole clue as a definition .
PB@5, I’ve amended the blog to reflect you comment
Roz@7 I suppose ADULTERY has to be an &lit, albeit with a little bit of a clunky definition. Though if any clue could involve double duty or cheating, then it would have to be this one.
A mix of challenging and easy clues. Good variety, but I found many of the clues were wordy, many of which in turn were difficult to understand
Liked DASH, ASTRINGENT.
I learned another word for gaol. What does it say about the UK that there is so much slang associated with crime and gaol?
Thanks Jason and scchua
Yes Peter @9 the whole clue is double-dealing and two-timing .
Agree with Martyn and missed a few of the short ones. I even thought of Auto and didn’t put it in because I couldn’t begin to think why it was the answer (other than being an anagram of a ABOUT minus B)
Not a favourite.
Thanks Jason and Scchua.
We’ve read all the comments (thanks all) but are really struggling with 5d. We know it’s a reverse anagram but can anyone explain exactly how it works?
The whole clue refers to AUTO , you can put bachelor B in it anywhere , then you can get ABOUT using an anagram , possibly .