Part of a race (3)
I believe the answer is:
lap
'part of a race' 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 "Once around the track" , "Track circuit; drink" , "Thighs, when seated" , "400 metres, typically" , "One round of the course" .)