Convexity of a road (6)
I believe the answer is:
camber
'convexity of a road' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for camber that I've seen before include "bend in road" , "Curvature of aircraft wing" , "Curve given to surface as of road" , "Slope on road surface" , "Slight tilt" .)