Pie cuts, essentially (5)
I believe the answer is:
radii
'pie cuts essentially' 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 "Lines from centre to perimeter of a circle" , "Halves of diameters" , "Centre-circumference lines" , "Veins of insect's wings" , "Half-diameters" .)