Russian ruler keeps eastern church in clemency (8)
I believe the answer is:
lenience
'clemency' is the definition.
(I've seen this in another clue)
'russian ruler keeps eastern church' is the wordplay.
'russian ruler' becomes 'lenin' (I've seen this in another clue).
'keeps' means one lot of letters goes inside another.
'eastern' becomes 'e'.
'church' becomes 'CE' (abbreviation for Church of England).
'lenin' placed around 'e' is 'lenien'.
'lenien'+'ce'='LENIENCE'
'in' acts as a link.
(Other definitions for lenience that I've seen before include "Quality of mercy in awarding punishment" , "Tolerance" , "indulgence" .)