"Slippery" swimmer (3)
I believe the answer is:
eel
'slippery swimmer' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for eel that I've seen before include "A sea snake" , "Slimy fish" , "Slippery person" , "Unagi, in a sushi bar" , "slipper" .)