Halves of diameters (5)
I believe the answer is:
radii
'halves of diameters' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for radii that I've seen before include "Lines from centre to perimeter of a circle" , "Lines drawn from a circle's centre" , "Halves of diameter" , "Half diameters" , "lines from a central part" .)