Selassie devotee overthrowing an emperor (5)
I believe the answer is:
rasta
'selassie devotee' is the definition.
'overthrowing an emperor' is the wordplay.
'overthrowing' is a reversal indicator.
'an' becomes 'a'.
'emperor' becomes 'tsar' (I've seen this before).
'a'+'tsar'='atsar'
'atsar' written backwards gives 'RASTA'.
(Other definitions for rasta that I've seen before include "Dreadlocked follower of Haile Selassie" , "Jamaican cultist" , "Jamaican sect member" , "Member of religious movement originating in Jamaica" , "Believer with dreadlocks" .)