A drug, which is out of this world (8)
I believe the answer is:
asteroid
'which is out of this world' is the definition.
(celestial body)
'a drug' is the wordplay.
'drug' becomes 'steroid' (type of drug).
'a'+'steroid'='ASTEROID'
(Other definitions for asteroid that I've seen before include "could be dangerous for astronauts" , "Small celestial body moving around the sun" , "Minor planet" , "Bits of rock that move around the heavens" , "Small rocky object orbiting the sun - store aid (anag)" .)