Pool exercise (4)
I believe the answer is:
swim
'pool exercise' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for swim that I've seen before include "Take a dip" , "Do the dog paddle, perhaps" , "At . . . . Two Birds by Flann O'Brien" , "Be immersed" , "progress through water" .)