Finish any race in the top 3 without any damage in career.
This one is tricky, and is best saved for once you get the hang of the controls and the maps. Also having a fast bike like the Blue Lightning dirt bike, which is unlocked later on in the game, will help you gain a good lead from the start, and hopefully stop the biggest pain of getting this achievement, the crash happy A.I.
Your damage is monitored in the upper right hand corner of the HUD, you are able to take lil scrapes and bumps, but if your HUD shows colour on your damage monitor then you will know you have been damaged and unable to gain the achievement.
Also, certain race types such as Head-to-Head and Endurance will not work. It must be a Circuit race.