Drug sustaining a heavenly body! (8)
I believe the answer is:
asteroid
'heavenly body' is the definition.
(asteroid is a kind of heavenly body)
'drug sustaining a' is the wordplay.
'drug' becomes 'steroid' (type of drug).
'sustaining' is a charade indicator (letters next to each other).
'steroid' after 'a' is 'ASTEROID'.
(Other definitions for asteroid that I've seen before include "circling rock" , "Small body orbiting sun" , "Small celestial body composed of rock and metal" , "Small heavenly body" , "Rocky body in orbit" .)