Father beginning to eat fish (4)
I believe the answer is:
pope
'fish' is the definition.
(I've seen this before)
'father beginning to eat' is the wordplay.
'father' becomes 'pop' (pop is a kind of father).
'beginning to' says to take the initial letters.
The first letter of 'eat' is 'e'.
'pop'+'e'='POPE'
(Other definitions for pope that I've seen before include "author" , "church leader in Rome" , "Poet - religious leader" , "Satirist" , "Head of Roman Catholic Church" .)