General road safety margin (6)
I believe the answer is:
leeway
'safety margin' is the definition.
(I've seen this before)
'general road' is the wordplay.
'general' becomes 'Lee' (Robert E. Lee - US revolutionary commander).
'road' becomes 'way' (I've seen this in other clues).
'lee'+'way'='LEEWAY'
(Other definitions for leeway that I've seen before include "Margin of safety" , "Permissible difference, allowing some freedom to move" , "Room for free movement" , "Allowable margin, latitude" , "Room for movement within limits" .)