Design extraterrestrial world (6)
I believe the answer is:
planet
'world' is the definition.
(a planet is a world)
'design extraterrestrial' is the wordplay.
'design' becomes 'plan' (synonyms).
'extraterrestrial' becomes 'et' (I've seen this in other clues).
'plan'+'et'='PLANET'
(Other definitions for planet that I've seen before include "one of seven in suite" , "Orbiting heavenly body" , "Eg, Mars, Venus" , "Earth, for example" , "Heavenly (or earthly!) body" .)