Sea god poisoned at sea (8)
I believe the answer is:
poseidon
'sea god' is the definition.
(Greek god of the sea)
'poisoned at sea' is the wordplay.
'at sea' is an anagram indicator.
'poisoned' with letters rearranged gives 'POSEIDON'.
(Other definitions for poseidon that I've seen before include "Poisoned the Greek god of the sea" , "I snooped to find Greek god of the sea" , "being able to create waves?" , "Greek sea-god" , "Olympian" .)