Authoritarian unit involved in unstable Atlanta riot (12)
I believe the answer is:
Here is my best explanation:
'authoritarian' is the definition.
'unit involved in unstable atlanta riot' is the subsidiary indication.
I cannot quite understand how this works, but
'unit' could be 'total' (total is a kind of unit) and 'total' is present in the answer.
This explanation may well be incorrect...
(Other definitions for totalitarian that I've seen before include "Of ultimate-State-control system", "Despotic", "One-party", "far from liberal", "Tyrannical", "(Of states) extremely dictatorial", "Allowing no opposition" and "Dictatorial".)