Filly's father
I believe the answer is:
'filly's father' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sire that I've seen before include "Foal's father" , "Old form of address to king - rise to it" , "Stallion; beget" , "Male parent of, say, horse" , "Male parent of a horse, for example" .)