A drug to get 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 drug' is the wordplay.
'drug' becomes 'steroid' (type of drug).
'a'+'steroid'='ASTEROID'
'to get' acts as a link.
(Other definitions for asteroid that I've seen before include "Small planet revolving round the sun" , "Any of the small planets revolving around the sun" , "Bits of rock that move around the heavens" , "space traveller" , "Small heavenly body" .)