Great fun from Goliath.
Thanks to Goliath for an enjoyable puzzle. A good steady solve.

ACROSS
1. One panics about the wings touching fog (8)
ALARMIST
ALAR (about the wings) touching MIST (fog)
5. Recommendation to see trailer: it’s gripping (6)
ADVICE
AD (trailer) + VICE (it’s gripping)
9. Quad redesigned with cute water channel (8)
AQUEDUCT
(QUAD with CUTE)* (*redesigned)
10. Dump is full, needs emptying. Shame having to go back (3-3)
FLY-TIP
F[ul]L (needs emptying) + PITY< (shame, <having to go back)
12. Machine producing fifty articles (5)
LATHE
L (fifty, Roman numeral) + A; THE (articles)
13. Not aware plain cover for Elvis is oddly missing (9)
OBLIVIOUS
OBVIOUS (plain) cover for [e]L[v]I[s] (oddly missing)
14. Suitable for each (6)
PROPER
16. Return of monarchy from abroad (7)
FOREIGN
OF< (<return) + REIGN (monarchy)
19. Standard accessories for events (7)
PARTIES
PAR (standard) + TIES (accessories)
21. Bedsit renovated with bathroom fixtures (6)
BIDETS
23. Both sides keeping little money to pursue a special messenger (9)
ARCHANGEL
R; L (both sides) keeping CHANGE (little money) to pursue A
25. Occasionally taking paid time out to say goodbye (5)
ADIEU
[p]A[i]D [t]I[m]E [o]U[t] (occasionally)
26. Refer to Goliath’s comeback as sickening (6)
EMETIC
(CITE (refer to) + ME (Goliath))< (<comeback)
27. Giving gifts, finish in debt (8)
ENDOWING
END (finish) + OWING (in debt)
28. Opportunity to sample special treats (6)
TASTER
29. Look at ship’s make-up (8)
EYELINER
EYE (look at) + LINER (ship)
DOWN
1. Maybe Moroccan landscape extremely suitable for growing crops (6)
ARABLE
ARAB (maybe Moroccan) + L[andscap]E (extremely)
2. Worship without a prayer? The preacher ultimately is a cheat (9)
ADULTERER
ADUL[a]TE (worship without A) + [praye]R [th]E [preache]R (ultimately)
3. Bug nestling in humid genitalia (5)
MIDGE
[hu]MID GE[nitalia] (nestling in)
4. Fool on the phone getting help (7)
SUCCOUR
“SUCKER” (fool, “on the phone”)
6. Act to save vital organ and it’s saved (9)
DELIVERED
DEED (act) to save LIVER (vital organ)
7. Interested in acquiring Ravel’s first prelude (5)
INTRO
INTO (interested in) acquiring R[avel] (‘s first)
8. Previously working as a model? That’s revealing (8)
EXPOSING
EX (previously) + POSING (working as a model)
11. Key turned up inside duffel coat (4)
CLEF
[duf]FEL C[oat]< (<turned up, inside)
15. Detectives told to transfer ownership (9)
PRIVATISE
“PRIVATE EYES” (detectives, “told”)
17. Perception of home schooling (9)
INTUITION
IN (home) + TUITION (schooling)
18. Dad’s back with mum? It’s looking likely (8)
APPARENT
PA< (dad, <back) with PARENT (mum?)
20. Herb sausage America dismissed (4)
SAGE
SA[usa]GE (USA (America) dismissed)
21. Times coverage of isolated rubbish (7)
BALONEY
BY (times) coverage of ALONE (isolated)
22. About food served up, it’s fast food (6)
BURGER
(RE (about) + GRUB (food))< (<served up)
24. Sea travel reportedly needs them (5)
CREWS
“CRUISE” (sea travel, “reportedly”) – semi &lit
25. Everyone is getting round to this island (5)
ATOLL
ALL (everyone) is getting round TO
I worried about clef/key. Music might be in G major and the treble clef — two very different things. I suppose the clef provides a “key” to what pitches are used, but this is a bit of a stretch, methinks. Is there another explanation?
This was good fun, only alar was new to me. I liked PRIVATISE.