Circles round radius segments (6)
I believe the answer is:
orbits
'circles' is the definition.
(orbiting is a kind of circling)
'round radius segments' is the wordplay.
'round' becomes 'o' (round can mean a round object).
'radius' becomes 'r' (abbreviation in geometry).
'segments' becomes 'bits'.
'o'+'r'+'bits'='ORBITS'
(Other definitions for orbits that I've seen before include "Whole rounds" , "Goes around like paths of planets" , "Circulates" , "Paths of satellites around planets" , "Elliptical courses of celestial objects" .)