Father joins eastern leader (4)
I believe the answer is:
pope
'leader' is the definition.
(pope is a kind of leader)
'father joins eastern' is the wordplay.
'father' becomes 'pop' (pop is a kind of father).
'joins' means one lot of letters go next to another.
'eastern' becomes 'e'.
'pop'+'e'='POPE'
(Other definitions for pope that I've seen before include "freshwater fish" , "Well-versed man" , "man of great authority" , "Head of Roman Catholic Church" , "Bishop of Rome" .)