Find fault with swimmer (4)
I believe the answer is:
carp
This clue is a double definition.
'find fault' is the first definition.
(I've seen this in another clue)
'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 "Fish - grouse" , "Raise petty objections about goldfish" , "Fish often kept in ponds" , "Swimmer" , "Fish < and beef" .)