A growth promoter for a heavenly body (8)
I believe the answer is:
asteroid
'heavenly body' is the definition.
(I know that asteroid is a type of minor planet)
'a growth promoter for a' is the wordplay.
'a growth promoter' becomes 'steroid' (steroids might grow muscles).
'for' means one lot of letters go next to another (I've seen this in other clues).
'steroid' put after 'a' is 'ASTEROID'.
(Other definitions for asteroid that I've seen before include "looking like a star?" , "Any of the small planets revolving around the sun" , "Small celestial body moving around the sun" , "Bits of rock that move around the heavens" , "Piece of rock that moves around the heavens" .)