Posted wrong information initially about authoritarian (8)
I believe the answer is:
despotic
'authoritarian' is the definition.
(synonyms)
'posted wrong information initially about' is the wordplay.
'wrong' indicates an anagram.
'initially' suggests taking the first letters.
'about' becomes 'c' (abbreviation for 'circa').
The initial letter of 'information' is 'i'.
'posted' anagrammed gives 'despot'.
'despot'+'i'+'c'='DESPOTIC'
(Other definitions for despotic that I've seen before include "of tyrant" , "'Cruel and oppressive, as a dictator (8)'" , "Autocratic" , "Referring to absolute power" , "Dictatorial" .)