This achievement requires you to complete a clean race with one of the cars from the pack, these cars are:Lancia Stratos - 70s (Rally)
Peugeot 205 T16 Evo 2 - Group B (Rally/Rallycross)
Citroen C4 Rallycross - Rallycross
Ford Mustang GT - Trailblazer (Modern)
Saturn Sky - Gymkhana
(This car doesnt count as its not available in a race)
You can unlock the achievement in the Dirt Tour, Singleplayer or Live Multiplayer (You cannot unlock this via a timetrial event)
The quickest way to do this is to start a Singleplayer Rallycross
at Monaco - Casino Square
, set it to 1 Lap and use the Citroen C4 Rallycross, seeing as you Do not
need to win let the AI pass you at the start line and then take the course easy making sure not to touch any walls or opponents.The achievement will pop at the loading screen after the race
Note: When trying for this the achievement did no unlock despite saying it had, i had to redo the achievement 4 times for it to successfully upload to xbox.com, so when the achievement unlocks load up your xbox profile
and check it has uploaded, if it has not close the game and restart then try again.
Here's a video to demonstrate.