Violent demonstrator (6)
I believe the answer is:
rioter
'violent demonstrator' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rioter that I've seen before include "He is taking part in a public affray" , "He runs amok in public" , "One engaged in a violent disturbance of the peace" , "One of a violent crowd" , "He's running amok in the street" .)