Circle, or segment (5)
I believe the answer is:
orbit
'circle' is the definition.
(orbiting is a kind of circling)
'or segment' is the wordplay.
'segment' becomes 'bit' (I've seen this before).
'or'+'bit'='ORBIT'
(Other definitions for orbit that I've seen before include "astronauts' course" , "Range of influence" , "Path through space" , "revolutionary course" , "Path of a planet or satellite" .)