Spokes (5)
I believe the answer is:
radii
'spokes' is the definition.
('radius' can be a synonym of 'spoke')
This is the entire clue.
(Other definitions for radii that I've seen before include "Lines drawn from a circle's centre" , "Halves of diameter" , "Lines from centre to perimeter of a circle" , "Two making up diameter" , "spoke with others" .)