Curve on road needs key traffic light (6)
I believe the answer is:
camber
'curve on road' is the definition.
'camber' can be an answer for 'curve' (I have seen 'Curve of road ' mean 'camber' so perhaps 'curve' could also mean 'camber'). I'm not sure about the 'on road' bit.
'key traffic light' is the wordplay.
'key' becomes 'c' (musical key).
'traffic light' becomes 'amber' (I've seen this in another clue).
'c'+'amber'='CAMBER'
'needs' is the link.
Can you help me to learn more?
(Other definitions for camber that I've seen before include "Slope on road surface" , "Convex shape of road surface" , "may appear on road" , "bend in road" , "Slight convexity, like the surface of a road" .)