Horse father (4)
I believe the answer is:
sire
'horse 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 "Ancestor" , "Rise for male parent (4)." , "Old form of address to king - rise to it" , "Male parent of an animal" , "Male parent of a horse, for example" .)