Slight convexity (6)
I believe the answer is:
camber
'slight convexity' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for camber that I've seen before include "Slight upward curve in road, wing etc" , "Slope on road surface" , "Slight convexity, like the surface of a road" , "Road surface curvature" , "bend in road" .)