Mock swimmer (3)
I believe the answer is:
cod
I believe this is a double definition.
'mock' is the first definition.
(cod can mean false or inauthentic)
'swimmer' is the second definition.
(cod is a kind of fish)
(Other definitions for cod that I've seen before include "Tasty fish or slangy nonsense" , "Initially, pay when you receive the goods" , "A joke of a fish?" , "method of payment" , "A white fish" .)