The clues are far too obscure for a 69 year-old Brit! I didn't understand references to US musicians. "Patisserie" is a pastry, or a shop selling them, not one who makes them. I'm afraid that I gave up.
People from the Rocky Mountains are quiet, oh quiet one (8)
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Across
9A
Cross a pair of hyenas with newlywed around a Hawaiian singer, familiarly (9)
10A
MDMA tab taken up after a decline (5)
11A
They're with Iggy, Larry, and Moe (7)
12A
Dead track of a freighter isn't out (7)
13A
Phoebe spans over a run (8)
15A
I heard a single individual (4)
16A
Spanish saint comes back to be a rapper (3)
18A
Savant puts together 13A13APhoebe spans over a run (8)[--------], 6D6DSmall California school's marketing push comes back to Lucy (5)[-----], and 24D24DKing messes around with "Bear" patissier (5)[-----] (9)
21A
Instead, I heard a paddle (3)
22A
Follow inside to be yourself (4)
23A
The poor sea churns inside pants (8)
25A
Yes, Leah cut up a strip of hair (7)
26A
President takes a cookie (7)
27A
Love for a vile, vile fruit (5)
28A
Cut vertical in half after you take out second indent to make it the same (9)
Down
1D
People from the Rocky Mountains are quiet, oh quiet one (8)
2D
Bros flip to be held in muscles' grips (7)
3D
Flamingo flying away from Florida gets taken in by families of the ruler of a labyrinthine land? (4, 5)
4D
Images of moments I heard, perhaps controversially? (4)
5D
Numbers surround stripped hectare of fruits (10)
6D
Small California school's marketing push comes back to Lucy (5)
7D
Mark Harmon in a frenzy after losing man to a goat (7)
8D
Kennel changes one temperature to another to produce an herb (6)