Outer planet (6)
I believe the answer is:
uranus
'outer planet' is the definition.
(I know that Uranus is a outer planet)
This is all the clue.
(Other definitions for uranus that I've seen before include "A major planet or the most ancient Greek god" , "Most ancient of Greek gods, and a major planet" , "Planet that's the butt of much wordplay" , "space traveller" , "Planet discovered in 1781 by Herschel" .)