Eighth planet from the sun (7)
I believe the answer is:
neptune
'eighth planet from the sun' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for neptune that I've seen before include "Third largest of the planets, and sea god" , "Planet, identified 1846" , "revolutionary body" , "Holst piece" , "Roman sea-god" .)