Another pleasing and tractable puzzle from Everyman. Once I got a foothold with some of the anagrams and the trademark initial letter clue, it fell out nicely. Which is how it should work, after all.
Abbreviations
cd cryptic definition
dd double definition
cad clue as definition
(xxxx)* anagram
anagrind = anagram indicator
[x] letter(s) removed
definitions are underlined
Across
1 Relief: Everyman finally grasped insult
SNUB
An insertion of N for the final letter of ‘Everyman’ in SUB. The insertion indicator is ‘grasped’.
4 Sprinkle powder on and wipe powder off
DUST
A dd. DUST a contronym: a word having two opposite meanings. Better known examples are cleave, sanction and fast.
8 Eg water treated: our rates unclear
NATURAL RESOURCE
(OUR RATES UNCLEAR)* with ‘treated’ as the anagrind.
11 Faulty odograph – no parking – bad driver
ROAD HOG
(ODOGRA[P]H)* with ‘faulty’ as the anagrind.
12 Innocent idyll – but more like a source of amusement, you say?
ARCADIA
A soundalike clue (‘you say’) for the entirely made-up word ARCADIER, which whimsically could mean ‘more like an amusement arcade’.
13 Repaired trustable counters
REBUTTALS
(TRUSTABLE)* with ‘repaired’ as the anagrind.
14 Several offices and a bar
BLOCK
A dd.
15 Type of approval from monks’ boss
PRIOR
Another dd.
16 When I’m working, you don’t see the point of me
SAFETY PIN
A cd.
19 Certain anorak granting owner undrenched life experience, primarily?
CAGOULE
The initial letters of the first seven words of the clue and a cad.
21 About to get fleeced? That’s rotten
CROOKED
A charade of C for circa and ROOKED.
22 Wastes time and sells dodgy lasagne?
FLOGS A DEAD HORSE
A dd cum cd. Everyman is suggesting that if lasagne were made illegally from horse meat rather than beef then this phrase might be a description of it, as well as a phrase meaning wasting your time. They eat horses, don’t they? And I make my lasagne with a mixture of pork and beef, but that’s by the by.
23 Whoa, accumulated prize money in rollover
STOP
A reversal of POTS.
24 Audibly expressed relief in half of LP
SIDE
A soundalike clue (‘audibly’) of SIGHED.
Down
2 Source of illumination, never-ending standard, the shrink
NORTH STAR
‘Never-ending’ is a last letter removal, so I think this is NOR[M], TH[E] and STAR[T]. Except START and ‘shrink’ are remote synonyms. ‘The sudden noise made him start/shrink.’ Happen I am barking up the wrong tree.
3 African country, not the last, then old boy heading North to see Italian city
BOLOGNA
A reversal (‘heading north’) of ANGOL[A] and OB.
4 Entertainers like Dereks Jarman and Jacobi?
DEEJAYS
Both those cultural figures had the initials DJ …
5 Much gusto I convey? Not I!
STOIC
Hidden in guSTO I Convey with an extended definition.
6 One who stops you going out at night?
SNORER
A cd.
7 Note: US rockers given sanctuary
REMARK
A charade of REM and ARK.
9 Then the boasting worked
THAT BEING SO
(THE BOASTING)* with ‘worked’ as the definition.
10 Communist angry with card game having bloomer
RED-HOT POKER
A charade of RED, HOT and POKER.
14 Hazard? Rubbish: that man had engaged
BETROTHED
A charade of BET, ROT and HE’D.
15 Calm provided among fleet
PACIFY
An insertion of IF in PACY. The insertion indicator is ‘among’.
16 Went in to dine: get a move on!
SPEED UP
An insertion of PEED in SUP. The insertion indicator is ‘in’.
17 Is a calf, implausibly, getting spa treatments?
FACIALS
(IS A CALF)* with ‘implausibly’ as the anagrind.
18 Approved some damn odd editorials
NODDED
Hidden in damN ODD EDitorials.
20 Distress increases on meeting Martian?
UPSET
A charade of UPS and ET.
Many thanks to Everyman for this Sunday’s puzzle.

Perhaps the reference in 22a is to the horse meat scandal of 2013 which made headlines in the UK. It has its own Wikipedia page. Lasagnes are mentioned!
Thanks for the blog, Pierre.