To earn this achievement that's called "Magic Second", you must finish 0.5 seconds ahead of an opponent in any race.
I actually got this one by sheer, dumb luck. While going after three stars in the World Tour mode at Costal Carnage, I somehow managed to stay ahead of several others as I went through the finish line in the last moments, apparently 0.5 seconds ahead. This achievement may take a couple of tries to get, perhaps even longer, but if you manage to stay ahead of someone within that seconds range as you cross the finish line of the final lap, the achievement should unlock. Also, if you're curious to know who I used, I picked Sonic and had the Balance mod equipped before going through the race and getting the achievement in the end. Also, if you are successful, the achievement unlocks after you meet the criteria and finished the race. As long as you are not dead last, you should be fine.
Have fun and good luck!