Thanks Hoskins. Witty surfaces here. Definitions are underlined in the clues.
Across
1 Master with gas released big noises (9)
MEGASTARS : Anagram of(… released) [MASTER plus(with) GAS].
Defn: VVIPs.
6 Man that must take pee in wood (5)
MAPLE : MALE(a man) containing(that must take) P(the letter “pee”).
9 Proper tune I chat about (9)
AUTHENTIC : Anagram of(… about) TUNE I CHAT.
10 In a bit of fluster after horse bolts (5)
LATER : “lather”(fluster/a state of agitation) minus(after … bolts) “h”(abbrev. for “heroin”/in slang, horse).
Defn: … of time more, as in “I’ll do that in a bit, after I’ve finished this”.
11 Superior evening meal son polished off (5)
UPPER : “supper”(the evening meal) minus(… polished off) “s”(abbrev. for “son”).
12 Quite angry about female star of The Deep? (9)
CROSSFISH : CROSS-ISH(angry + suffix denoting “quite”/to some degree) containing(about) F(abbrev. for “female”).
Defn: …, ie. a starfish.
13 Pretty flirty cow might do this, I’m told (7)
EYEABLE : Homophone of(…, I’m told) “Eye a bull”(whimsically, what a randy cow might do).
15 Pressure to split up old sort and a model (7)
REPLICA : P(symbol for “pressure” in physics) contained in(to split up) RELIC(an old sort/an article from olden times) plus(and) A.
18 Ace part of speech produces memorable phrase (7)
PROVERB : PRO(short for “professional”/an expert/an ace at some activity) + VERB(a part of speech, in grammar).
20 Fruit bats (7)
BANANAS : Double defn: 2nd: Mad.
21 Mafioso cut down in wicked vendetta’s finale (9)
GOODFELLA : FELL(to cut down, a tree, say) contained in(in) GOOD(excellent/in slang, “wicked”) + last letter of(…’s finale) “vendetta“.
Defn: Americanism for a member of the Mafia crime organisation/a Mafioso.
23 Hate violent crime associated with men (5)
ABHOR : ABH(abbrev. for “actual bodily harm”, a criminal offence in the UK) plus(associated with) OR(abbrev. for “other ranks”/non-commissioned men in the military).
25 Woman I married shows spirit (5)
ANIMA : Hidden in(… shows) “Woman I married“.
26 Miserable function a performer gate-crashes (9)
SATURNINE : SINE(a trigonometric function equal to a ratio involving two sides of a right-angled triangle) containing(… gate-crashes) [A + TURN(a performer/one who gives a turn/a short performance)].
27 Head for New Zealand with Scandinavian (5)
SWEDE : Double defn: 1st: Slang in New Zealand for one’s head, so named after the vegetable, presumably.
28 Most important horse soldier at the front (9)
PARAMOUNT : MOUNT(a horse that one is, well, mounted on) placed after(… at the front) PARA(short for a paratrooper/a soldier who goes into combat by parachuting from airplanes).
Down
1 Perhaps fingers in the air meet requirements (7,2)
MEASURE UP : MEASURE(an example/perhaps of a measure of spirits in a glass is the finger, based on the breadth of an unspecified finger) + UP(in the air/aloft).
2 Rise and dress (3-2)
GET-UP : [GET UP](to rise from a lower or sitting position).
3 Ever entered into firm with capacity to take direction? (9)
STEERABLE : E’ER(literary form of “ever”/always) contained in(entered into) STABLE(firm/steady).
4 Diplomat committed to seeing someone shortly (7)
ATTACHE : “attached”(committed to seeing someone/already in a relationship) minus its last letter(shortly).
5 Help victim of a con artist in hearing (7)
SUCCOUR : Homophone of(… in hearing) “sucker”(victim of a con artist/swindler).
6 They smuggle another’s drugs in shoes (5)
MULES : Double defn.
7 I paint car in a slap-dash way for aristocrat (9)
PATRICIAN : Anagram of(… in a slap-dash way) I PAINT CAR.
8 On occasion, lairy dons dished the dirt (5)
EARTH : 2nd and 4th letters of(On occasion) “lairy” contained in(dons) anagram of(dished) THE. The construction is a bit awkward, I thought.
14 Use less MDMA? Come on, silly, one’s consumed by it! (9)
ECONOMISE : Anagram of(…, silly) [ E(abbrev. for the drug, ecstasy, the chemical compound methylenedioxy?methamphetamine abbreviated as MDMA) + COME ON] containing(… consumed by it) I(Roman numeral for one)‘S .
16 Another pagan term? (9)
PENTAGRAM : Anagram of(Another) PAGAN TERM.
Answer: A symbol/a term of faith used by Wiccans, modern Pagans). A WIWD (wordplay intertwined with definition) clue.

17 Spongy dessert knight tucks into after sailor (9)
ABSORBENT : [SORBET(a frozen dessert of flavoured water) containing(… tucks into) N(abbrev. for “knight”, a piece in chess)] placed after(after) AB(abbrev. for “able-bodied sailor”).
19 Drink following party – that’s a mistake (5,2)
BALLS UP : SUP(to drink slowly) placed after(following) BALL(a formal party involving dancing).
Defn: …, aka a screw-up.
20 I am heading to soirée wearing straw hat (7)
BOASTER : 1st letter of(heading to) “soirée” contained in(wearing) BOATER(a straw hat).
Defn: Loosely, the clue could be a statement made by one bragging about his/her social status.
A WIWD clue.
21 Good girl? Crystal could be one! (5)
GLASS : G(abbrev. for “good”) + LASS(a young woman/a girl, whose name could be Crystal!).

22 Fellow with impotent lover (5)
FLAME : F(abbrev. for “fellow”) plus(with) LAME(impotent/defective).
24 Homework at inferior Karachi uni initially takes little work (5)
HAIKU : 1st letters, respectively, of(… initially) “Homework at inferior Karachi uni“.
Defn: A short poem in the style of this particular form of Japanese verse:
A stone at its core,
this snowball’s the porcelain
knob on winter’s door.
Paul Muldoon
Or the final word:
I hate writing hai-
ku because you only get
seventeen syllab…
Unknown
I liked the ‘flirty cow’ at 13a and the not obvious ‘In a bit’ def for 10a. The surface for FLAME and the &lit PENTAGRAM were also good. Didn’t know SWEDE for ‘head’ in NZ and missed the parsing of EARTH which was my last in. I just had ‘I am’ as the def for 20d, with the rest of the clue making up the wordplay, though wasn’t really convinced about it.
A pleasant bit of fun for a Sunday afternoon.
Thanks to Hoskins and scchua
Thanks to Hoskins & scchua. I’m not sure why Collins lists the slang use of swede=head as NZ. My father, who never left GB and indeed only left Yorkshire/Derbyshire a handful of times used to refer to the act of putting one’s head on a pillow as “bashing your swede”
All good fun except that 8d seemed a bit of a strange clue.
A special mention for 12a which brought back some happy memories of Jacqueline Bisset in her wet t-shirt in The Deep.
Many thanks to Hoskins and to scchua.
I didn’t know the starfish and took some convincing that 13a, funny as it is, was a real word. 8d scored an ‘umm’ here.
Much enjoyment to be found elsewhere in the puzzle and it was the 19d mistake that produced the widest grin – been there, done that!
Many thanks to Hoskins and to scchua for the blog.
Hoskins in good and amusing form. CROSSFISH last one in, clue meant nothing to me. Am also going to check if 13A is in any dictionaries, unless somebody posts before I get around to it. Liked 1 and 26A, 16 and 17D.
The explanation of 14D has omitted the “come on” anagram.
Thanks to Hoskins and scchua.
Thanks gwep. Oversight corrected.
One or two we couldn’t parse, such as 8dn. 13ac was our LOI; presumably it’s in some dictionaries since we got it from a wordfinder.
Otherwise, all very good; an enjoyable solve.
Thanks, Hoskins and scchua.
I always love a Hoskins puzzle, and certainly found plenty of his characteristic humour to enjoy here.
Met the CROSSFISH for the first time and similarly had to trust that EYEABLE was a real word. The New Zealand part of SWEDE meant I wasn’t quite sure that I wasn’t meowing into the wrong bush with that one.
There’s no point listing my highlights as that would be much of the puzzle.
Thanks Hoskins and scchua.