Crowd disperser (4,3)
I believe the answer is:
tear gas
'crowd disperser' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tear gas that I've seen before include "Riot control means" , "Riot control weapon" , "Riot control agent" , "'It can be used to disperse crowds (4,3)'" , "Airlike substance that irritates the eyes, used in riots" .)