A bodybuilder or space traveller (8)
I believe the answer is:
asteroid
'space traveller' is the definition.
(asteroids travel through space)
'a bodybuilder' is the wordplay.
'bodybuilder' becomes 'steroid' (as in the type of drug).
'a'+'steroid'='ASTEROID'
'or' acts as a link.
(Other definitions for asteroid that I've seen before include "Lump of rock orbiting the sun" , "Small rocky object orbiting the sun - store aid (anag)" , "Ceres, say" , "Rocky body in orbit" , "Any of the small planets revolving around the sun" .)