Slimmer swimmer (3)
I believe the answer is:
eel
'slimmer 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 "Large elver" , "Slippery swimmer (may be jellied)" , "Slippery person" , "Snaky fish" , "Snake-like sea creature" .)