The eighth planet from the Sun (7)
I believe the answer is:
neptune
'the eighth planet from the sun' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for neptune that I've seen before include "Poseidon" , "King of the sea" , "Remote planet" , "Third largest of the planets, and sea god" , "One distanced from The Sun" .)