Tyrant posted abroad (6)
I believe the answer is:
despot
'tyrant' is the definition.
(synonyms)
'posted abroad' is the wordplay.
'abroad' indicates an anagram (abroad can mean moving around).
'posted' with letters rearranged gives 'DESPOT'.
(Other definitions for despot that I've seen before include "Caesar for one" , "One exploiting rule" , "Gaddafi, say" , "Autocrat" , "Cruel and oppressive dictator" .)