Sea god’s heavenly body (7)
I believe the answer is:
neptune
I believe this clue is a double definition.
'sea god's' is the first definition.
(thesaurus)
'heavenly body' is the second definition.
(Neptune is an example)
(Other definitions for neptune that I've seen before include "Roman god of the sea, with trident" , "Roman sea-god" , "Holst piece" , "Third largest of the planets, and sea god" , "Holst movement?" .)