I'd act to overturn republican despot (8)
I believe the answer is:
dictator
'despot' is the definition.
(I've seen this before)
'i'd act to overturn republican' is the wordplay.
'overturn' indicates an anagram.
'republican' becomes 'R' (abbreviation in US politics).
'id'+'act'+'to'='idactto'
'idactto' is an anagram of 'dictato'.
'dictato'+'r'='DICTATOR'
(Other definitions for dictator that I've seen before include "Hitler was one, like Stalin and Franco" , "Such as Saddam Hussein" , "person in charge? Absolutely!" , "Tyrant like Mussolini or Franco" , "Tyrant or oppressor" .)