This is just as easy as it seems. You just have to win a race. But in dirt racing, "winning a race" is when you finish first in the A-main. So you must qualify, run the heats, and the qualfying sprints, then you enter into the A-Main. You can do this in quick race or career. Not sure if it works in multiplayer, but just a quick race with a short race length on the easy difficulty will surely lock this cheevo up for you. The cheevo should pop after exiting the post race screen, and returning to the main menu. Happy hunting!