Tyrants posted away on Saturday (7)
I believe the answer is:
despots
'tyrants' is the definition.
('despot' can be a synonym of 'tyrant')
'posted away on saturday' is the wordplay.
'away' indicates an anagram (letters move away from their original places).
'on' is an insertion indicator (as in clothing 'on' a person).
'saturday' becomes 's'.
'posted' with letters rearranged gives 'depots'.
'depots' going around 's' is 'DESPOTS'.
(Other definitions for despots that I've seen before include "Autocrats" , "Tyrants" .)