American police hit heavily (4)
I believe the answer is:
swat
This clue is a double definition.
'american police' is the first definition.
'hit heavily' is the second definition.
(Other definitions for swat that I've seen before include "'Twas quite a smack" , "Hit (flies, etc)" , "deliver blow" , "Hit (insect) sharply" , "Crush (eg, a fly)" .)