Width of a circle (8)
I believe the answer is:
diameter
'width of a circle' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for diameter that I've seen before include "Maximum distance across circle" , "Circle's width" , "It's equal to twice the radius" , "Line crossing centre of sphere" , "Remade it to width of a circle" .)