Criminal ring riots regularly (6)
I believe the answer is:
bandit
'criminal' is the definition.
(bandit is a kind of criminal)
'ring riots regularly' is the wordplay.
'ring' becomes 'band' ('band' can be a synonym of 'ring'**).
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternating letters of 'riots' are 'it'.
'band'+'it'='BANDIT'
(Other definitions for bandit that I've seen before include "Outlaw, robber" , "Thief" , "Enemy aircraft" , "Outlawed robber" , "(One-armed?) outlaw" .)