Swimming workout units (4)
I believe the answer is:
laps
'swimming workout units' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for laps that I've seen before include "Scoops up with the tongue" , "Circuits of a racetrack" , "Drinks; circuits" , "Pals doing rounds of the track" , "Circuits of track" .)