Brief special vehicle branch of the police (6,5)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

FLYING (brief, as in a “flying visit” = not staying for a long time) + S (special) + QUAD (I don’t think I’ve seen this by itself as a name for a vehicle – but it could be short for quad bike, quadcopter drone, or various four-wheeled or four-passenger vehicles).