Sinuous swimmers (4)
I believe the answer is:
eels
'sinuous 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 "Slippery creatures" , "Snakelike fishes" , "Snakelike fish" , "Slithery fish" , "They have long bodies and very small fins" .)