A bodybuilder’s 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 bodybuilder's' is the wordplay.
'bodybuilder' becomes 'steroid' (as in the type of drug).
'a'+'steroid'='ASTEROID'
(Other definitions for asteroid that I've seen before include "Piece of rock that moves around the heavens" , "could be dangerous for astronauts" , "Small heavenly body" , "Rocky body in orbit" , "way-out rock" .)