Simply wreck a cop! A cop can be wrecked by something such as pushing them into a wall by turning your car towards them if they are running parallel to you, or doing a PIT maneuver where you use one of the front corners of your car to push into the side of the rear of the cop's car and cause him to spin and (hopefully) slam into a wall or obstacle or another car and be taken down.
This can be unlocked in Single Player or Multiplayer. In Single Player it is unlockable in non-event play (such as starting a chase by breaking laws, i.e. speeding) or in a race of some kind.