A bodybuilder — one’s out of this world! (8)
I believe the answer is:
asteroid
'one's out of this world' is the definition.
'a bodybuilder' 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 "Small celestial body composed of rock and metal" , "space traveller" , "looking like a star?" , "way-out rock" , "Small planet orbiting the Sun" .)