Jellied fish (3)
I believe the answer is:
eel
'jellied fish' 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 "Unagi, in a sushi bar" , "Moray or electric, for instance" , "Slimy fish" , "Slippery swimmer (may be jellied)" , "Almost scaleless fish" .)