A quick swim (3)
I believe the answer is:
dip
'a quick swim' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for dip that I've seen before include "may be lucky at the fete?" , "Sink; thick sauce" , "Drop" , "Immerse briefly in liquid" , "Thick sauce; brief swim" .)