Son's beginning to anger father (4)
I believe the answer is:
sire
'father' is the definition.
(I know that father can be written as sire)
'son's beginning to anger' is the wordplay.
'beginning to' indicates taking the first letters.
'anger' becomes 'ire' (ire is intense rage).
The initial letter of 'sons' is 's'.
's'+'ire'='SIRE'
(Other definitions for sire that I've seen before include "Male parent of, say, horse" , "Make children" , "Old form of address to king - rise to it" , "Male parent of a horse, for example" , "Generator" .)