Quarter of a circle (8)
I believe the answer is:
quadrant
'quarter of a circle' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for quadrant that I've seen before include "One fourth of circle" , "Quarter-circle" , "Quarter of a circle or of its circumference" , "Quarter sector of circle" , "Half of a semicircle" .)