Payment method for swimmer (3)
I believe the answer is:
cod
I believe this is a double definition.
'payment method' is the first definition.
(cash on delivery)
'swimmer' is the second definition.
(type of fish)
'for' is the link.
(Other definitions for cod that I've seen before include "A foolish sort of a fish" , "Fish; fake (colloq.)" , "faked" , "Cash on delivery for hoax" , "Initially, pay when you get the goods" .)