A Russian leader set about religious follower (5)
I believe the answer is:
rasta
'religious follower' is the definition.
(I've seen this before)
'a russian leader set about' is the wordplay.
'russian leader' becomes 'tsar' (I've seen this in another clue).
'set about' says the letters should be written backwards.
'a'+'tsar'='atsar'
'atsar' written backwards gives 'RASTA'.
(Other definitions for rasta that I've seen before include "Jamaican believer" , "One following religion" , "Member of cult worshipping Haile Selassie" , "sectarian worshipper" , "Jamaican cultist" .)