Line from the centre to the edge of a circle (6)
I believe the answer is:
radius
'line from the centre to the edge of a circle' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for radius that I've seen before include "Line between the centre and circumference of a circle" , "Distance from centre to circumference of circle" , "Straight line from circle's centre to circumference" , "It runs from the centre to the circumference" , "Centre-circumference length" .)