Go for a dip (4)
I believe the answer is:
'go for a dip' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for swim that I've seen before include "To be in the .... is knowing what is going on" , "Do the dog paddle, perhaps" , "At . . . . Two Birds by Flann O'Brien" , "Crawl, say" , "Ooze out; show lots of (confidence)" .)