Fish straying aimlessly (8)
I believe the answer is:
stingray
'fish' is the definition.
(type of fish)
'straying aimlessly' is the wordplay.
'aimlessly' is an anagram indicator (the letters rearranged chaotically or aimlessly).
'straying' anagrammed gives 'STINGRAY'.
(Other definitions for stingray that I've seen before include "Large venomous fish -- it's angry (anag)" , "Fish with venomous tail" , "Flatfish with barbed spine" , "Large fish with tail spine" , "menace in the water" .)