Math measurements (5)
I believe the answer is:
radii
'math measurements' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for radii that I've seen before include "Half-diameters" , "Veins of insect's wings" , "Lines from a circle's centre to the circumference" , "Spokes of wheel - bones of arm" , "Lines from the centre to the circumference of a circle" .)