A drug rock (8)
I believe the answer is:
asteroid
'rock' is the definition.
(an asteroid is a rock in space)
'a drug' is the wordplay.
'drug' becomes 'steroid' (type of drug).
'a'+'steroid'='ASTEROID'
(Other definitions for asteroid that I've seen before include "Small celestial body moving around the sun" , "Bits of rock that move around the heavens" , "Piece of rock that moves around the heavens" , "Small body orbiting sun" , "circling rock" .)