Such drivers dash or go mad (4,4)
I believe the answer is:
road-hogs
'such drivers' is the definition.
(road hogs are reckless drivers)
'dash or go mad' is the wordplay.
'mad' indicates an anagram.
'dash'+'or'+'go'='dashorgo'
'dashorgo' anagrammed gives 'ROAD-HOGS'.
(Other definitions for road-hogs that I've seen before include "They think they own the road" , "Selfish drivers" , "They think they own the highway" .)