Single circuit (3)
I believe the answer is:
lap
'single circuit' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lap that I've seen before include "400 metres, typically" , "One round of the course" , "Circuit of racetrack" , "A circuit of a racecourse" , "Thighs, when seated" .)