Father beginning to seethe with anger (4)
I believe the answer is:
sire
'father' is the definition.
(I know that father can be written as sire)
'beginning to seethe with anger' is the wordplay.
'beginning to' says to take the initial letters.
'with' is a charade indicator (letters next to each other).
'anger' becomes 'ire' (ire is intense rage).
The first letter of 'seethe' is 's'.
's'+'ire'='SIRE'
(Other definitions for sire that I've seen before include "Male parent of a horse, for example" , "Bear" , "Ancestor" , "Male parent of, say, horse" , "Generator" .)