Crowd disturbance -- island in decay (4)
I believe the answer is:
riot
'crowd disturbance' is the definition.
(I've seen this in another clue)
'island in decay' is the wordplay.
'island' becomes 'i' (geographical abbreviation).
'in' is an insertion indicator.
'decay' becomes 'rot' (rot can mean to decay).
'i' put into 'rot' is 'RIOT'.
(Other definitions for riot that I've seen before include "Public act of violence by an unruly mob" , "Disturbance by unruly mob" , "Crowd violence" , "Take to the streets" , "Dazzling display - revelry" .)