Mob scene (4)
I believe the answer is:
'mob scene' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for riot that I've seen before include "Public act of violence by an unruly mob" , "Wild revelry" , "unrestrained revelry" , "Lavish display (of colour)" , "Violent crowd disturbance" .)