Aquarium denizen (3)
I believe the answer is:
eel
'aquarium denizen' 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 "Unagi, in a sushi bar" , "User of electric organ" , "Snaky fish" , "Creature like conger or moray" , "A sea snake" .)