Adult elver (3)
I believe the answer is:
eel
'adult elver' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for eel that I've seen before include "Proverbially slippery fish" , "Fish in the Lee?" , "Almost scaleless fish" , "Moray or electric, for instance" , "elusive person" .)