Half-diameters (5)
I believe the answer is:
radii
'half-diameters' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for radii that I've seen before include "parts for arms" , "Spokes of a circle" , "Veins of insect's wings" , "Lines between the center and circumference of a circle" , "Half diameters" .)