Public disturbance (6)
'public disturbance' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for riot that I've seen before include "Unruly assembly" , "Civil unrest" , "Uproar" , "Civil disorder" , "A public affray or disturbance" .)
(Other definitions for affray that I've seen before include "Breach of the peace by public rioting" , "Disturbance in public place" , "Breach of the peace; brawl, fight" , "Disturbance of the peace" , "Brawl, disturbance" .)