Unrest as corruption besets one (4)
I believe the answer is:
riot
'unrest' is the definition.
(a riot is an instance of unrest)
'corruption besets one' is the wordplay.
'corruption' becomes 'rot' (I've seen this in another clue).
'besets' indicates putting letters inside (beset can mean to surround).
'one' becomes 'i' (Roman numeral).
'rot' placed around 'i' is 'RIOT'.
'as' is the link.
(Other definitions for riot that I've seen before include "A public disturbance by crowd" , "Violent crowd disturbance" , "Civil disturbance" , "unrestrained revelry" , "Wild revelry" .)