Ultimately, road rage becomes menace (6)
I believe the answer is:
danger
'menace' is the definition.
(I've seen this before)
'ultimately road rage' is the wordplay.
'ultimately' indicates one should take the final letters.
'rage' becomes 'anger' (I've seen this before).
The last letter of 'road' is 'd'.
'd'+'anger'='DANGER'
'becomes' is the link.
(Other definitions for danger that I've seen before include "Red flag, for example" , "Menace" , "Peril, hazard" , "Peril, jeopardy" , "Garden (anag) - peril" .)