Serpentine swimmer (3)
I believe the answer is:
eel
'serpentine swimmer' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for eel that I've seen before include "Slippery creature" , "Moray or electric, for instance" , "Teleost fish" , "slipper" , "Apode" .)