Heavenly body, not a fat-soluble compound (7)
I believe the answer is:
steroid
'fat-soluble compound' is the definition.
(I've seen this in another clue)
'heavenly body not a' is the wordplay.
'heavenly body' becomes 'asteroid' (asteroid is a kind of heavenly body).
'not' suggests deleting specific letters.
'asteroid' with 'a' taken out is 'STEROID'.
(Other definitions for steroid that I've seen before include "Body-building drug" , "Hormone perhaps" , "Controversial body-builder" , "might be abused in gym" , "growth medication" .)