Skinny swimmers (4)
I believe the answer is:
eels
'skinny swimmers' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for eels that I've seen before include "Food for Cockney" , "(Slippery) fish" , "Odd fish found in the lees" , "Long thin fish" , "Smooth fishes" .)