Road surface curvature (6)
I believe the answer is:
camber
'road surface curvature' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for camber that I've seen before include "Curve given to surface as of road" , "Upward curve of a road surface" , "Slope on road surface" , "Slight tilt" , "Slight upward curve in road, wing etc" .)