Acts of violence by unruly mobs (5)
I believe the answer is:
riots
'acts of violence by unruly mobs' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for riots that I've seen before include "Protests" , "Wild disturbances by crowds" , "Public disturbances" , "Acts of violence by unruly mobs" , "Violent disturbances" .)