Father, key church leader (4)
I believe the answer is:
pope
'church leader' is the definition.
(I've seen this before)
'father key' is the wordplay.
'father' becomes 'pop' (pop is a kind of father).
'key' becomes 'e' (I've seen this in other clues).
'pop'+'e'='POPE'
(Other definitions for pope that I've seen before include "Catholic leader" , "author" , "Pontiff" , "Poet - religious leader" , "church leader in Rome" .)