Father, eastern church leader (4)
I believe the answer is:
pope
'church leader' is the definition.
(I've seen this before)
'father eastern' is the wordplay.
'father' becomes 'pop' (pop is a kind of father).
'eastern' becomes 'e'.
'pop'+'e'='POPE'
(Other definitions for pope that I've seen before include "Head of RC church" , "fisherman's successor" , "Poet - religious leader" , "Catholic leader" , "St Peter was the first" .)