Convexity of a road (6)
I believe the answer is:
camber
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">convexity of a road</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)