Regal forms of address for fathers (5)
I believe the answer is:
sires
'for fathers' is the definition.
('sire' can be a synonym of 'father')
'regal forms of address' is the wordplay.
I cannot quite see how this works, but
'address' could be 'sir' (I have seen 'formal address' mean 'sir' so perhaps 'address' could also mean 'sir') and 'sir' is found within the answer.
The remaining letters 'es' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sires that I've seen before include "Begets" , "Fathers" .)