Go down for a swim (3)
I believe the answer is:
dip
I believe this clue is a double definition.
'go down' is the first definition.
(I've seen this before)
'a swim' is the second definition.
(dip is a kind of swim)
'for' acts as a link.
(Other definitions for dip that I've seen before include "Move downwards" , "Dunk" , "Go down" , "Plunge briefly in liquid" , "Slope downwards" .)