Circuit of a racetrack (3)
I believe the answer is:
lap
'circuit of a racetrack' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for lap that I've seen before include "Scoop up with the tongue" , "One round of the course" , "Knees" , "Circuit (of a race track)" , "Thighs, when seated" .)