Find fault with a swimmer (4)
I believe the answer is:
carp
This clue is a double definition.
'find fault' is the first definition.
(I've seen this before)
'a swimmer' is the second definition.
(carp is a kind of fish)
'with' acts as a link.
(Other definitions for carp that I've seen before include "Pond fish" , "Fish - complain" , "Fish - complaint" , "Moan; fish" , "Large freshwater fish" .)