A drug forming rock, perhaps (8)
I believe the answer is:
asteroid
'rock perhaps' is the definition.
(rock in space)
'a drug' is the wordplay.
'drug' becomes 'steroid' (type of drug).
'a'+'steroid'='ASTEROID'
'forming' is the link.
(Other definitions for asteroid that I've seen before include "circling rock" , "Rocky body in orbit" , "Lump of rock orbiting the sun" , "Small celestial body moving around the sun" , "Ceres, say" .)