Risk a couple of sharp bends, challenging corners (6)
I believe the answer is:
hazard
'risk' is the definition.
(synonyms)
'a couple of sharp bends challenging corners' is the wordplay.
'couple of sharp bends' becomes 'Z' (the letter Z is a line with two sharp bends).
'challenging' becomes 'hard' (similar in meaning).
'corners' means one lot of letters goes inside another.
'a'+'z'='az'
'az' inserted within 'hard' is 'HAZARD'.
(Other definitions for hazard that I've seen before include "Obstacle, source of danger" , "Venture" , "Risk; guess" , "Something dangerous, such as black ice?" , "Potential danger" .)