This will likely come with completing the Career Mode. Destroying a police car simply means eliminating it. This is quite easy to obtain as the police will chase you a lot of the time and they tend to pull up slowly right beside you, making for an easy ram into the wall. If you use the method mentioned in the "Petrol Head" achievement, you will get this as your unattended car will destroy several police cars on its way to finishing the event, without you even playing. All events can have police cars, except Cop Chase events, in which you ARE the police.