It's mad to negotiate the curve in the road (5,3,4)
I believe the answer is:
round the bend
'it's mad' is the definition.
(I've seen this before)
'negotiate the curve in the road' is the wordplay.
'negotiate' becomes 'round' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'curve in the road' becomes 'bend' (I've seen this in another clue).
'round'+'the'+'bend'='ROUND THE BEND'
'to' is the link.
Can you help me to learn more?
(Other definitions for round the bend that I've seen before include "Crazy (informal)" , "Slightly mad" , "hidden from view?" , "Nuts" , "Crackers" .)