Half a diameter (6)
I believe the answer is:
radius
'half a diameter' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for radius that I've seen before include "Straight line from circle's centre to circumference" , "Half a circle's diameter" , "Line from centre to edge of a circle" , "Bone of forearm - distance from centre" , "Straight line from centre to perimeter of a circle" .)