Large fish (5,3)
There are a few possibilities for this:
eagle ray
(Another definition for eagle ray that I've seen is " Fish".)
(Other definitions for stingray that I've seen before include "Fish with venomous tail" , "menace in the water" , "Poisonous spined fish" , "Large fish with tail spine" , "Dangerous fish" .)
'large fish' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sturgeon that I've seen before include "provider of delicacy?" , "The fish that yields caviar" , "Main resident" , "Fish - got nurse (anag)" , "So urgent to find caviar-bearing fish" .)