Serpentine swimmers (4)
I believe the answer is:
eels
'serpentine 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 "Else (anag.)" , "Else they're fish" , "Elongated fish" , "Snakelike fish" , "Elongated fishes" .)