Slippery swimmers (4)
I believe the answer is:
eels
'slippery swimmers' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for eels that I've seen before include "Smooth fishes" , "Fish (that may be jellied)" , "Sea snakes among lees" , "Conger --; jellied --" , "Jellied --, traditional dish" .)