Lines in circles (5)
I believe the answer is:
radii
'lines in circles' 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 drawn from a circle's centre" , "Lines from centre to perimeter of a circle" , "Spokes of wheel - bones of arm" , "Lines from the centre to the circumference of a circle" , "Veins of insect's wings" .)