Fish that may be jellied (4)
I believe the answer is:
eels
'fish that may be jellied' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for eels that I've seen before include "Conger --; jellied --" , "Slippery types" , "Slithery fish" , "Else they're fish" , "Feels small imprisoning water creatures" .)