Tyranny of a tsar not working with militia (15)
I believe the answer is:
totalitarianism
'tyranny' is the definition.
(synonyms)
'a tsar not working with militia' is the wordplay.
'working with' indicates anagramming the letters (working with the letters).
'a'+'tsar'+'not'='atsarnot'
'atsarnot' with its letters anagrammed with 'militia' makes 'TOTALITARIANISM'.
'of' is the link.
(Other definitions for totalitarianism that I've seen before include "Dictatorship" , "Allowing no opposition" .)