Roman sea god (7)
I believe the answer is:
neptune
'roman sea god' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for neptune that I've seen before include "Planet, identified 1846" , "Poseidon" , "Roman god of the sea, with trident" , "Roman sea god - planet" , "one of the Planets" .)