Fish straying at sea (8)
I believe the answer is:
stingray
'fish' is the definition.
(stingray is a kind of fish)
'straying at sea' is the wordplay.
'at sea' is an anagram indicator.
'straying' with letters rearranged gives 'STINGRAY'.
(Other definitions for stingray that I've seen before include "Large venomous fish -- it's angry (anag)" , "Dangerous fish" , "TV show" , "Marine creature with venomous tail" , "swimmer" .)