Slender swimmers (4)
I believe the answer is:
eels
'slender 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 "Long, fleshy fish" , "Else they're fish" , "Anguillae" , "Snakelike fishes" , "Congers" .)