Jamaican sectarian overthrows an emperor (5)
I believe the answer is:
rasta
'jamaican sectarian' is the definition.
(I've seen this before)
'overthrows an emperor' is the wordplay.
'overthrows' shows that the letters should be reversed in order.
'an' becomes 'a'.
'emperor' becomes 'tsar' (I've seen this before).
'a'+'tsar'='atsar'
'atsar' back-to-front is 'RASTA'.
(Other definitions for rasta that I've seen before include "Follower of Rastafarianism" , "Religious type" , "Member of WI religion" , "religious follower" , "Member of cult worshipping Haile Selassie" .)