Greek god of the sea (8)
I believe the answer is:
poseidon
'greek god of the sea' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for poseidon that I've seen before include "Greek god of the sea and earthquakes" , "underwater missile" , "Controller of the main" , "Sea Lord" , "Olympian" .)