Slippery fishes (4)
I believe the answer is:
eels
'slippery fishes' 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 "Snakes wriggling in lees" , "Slippery customers" , "They swim" , "Long thin fish" , "Anguillae" .)