That's a moray (3)
I believe the answer is:
eel
'that's a moray' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for eel that I've seen before include "Slippery type" , "User of electric organ" , "Ribbon-like fish" , "Moray or electric, for instance" , "elusive person" .)