A lot of mist on a road is a concern for drivers (6)
I believe the answer is:
hazard
'a concern for drivers' is the definition.
(as in a road hazard)
'a lot of mist on a road' is the wordplay.
'a lot of' means to remove the last letter (I've seen 'lots of' mean this (the majority of the word)).
'mist' becomes 'haze' (haze can mean a mist).
'on' is a charade indicator (letters next to each other).
'road' becomes 'rd' (abbreviation).
'haze' with its last letter taken away is 'haz'.
'haz'+'a'+'rd'='HAZARD'
'is' acts as a link.
(Other definitions for hazard that I've seen before include "Danger; gamble" , "Dice game" , "Venture" , "Potential danger" , "Golf course obstacle" .)