Filly's father (4)
I believe the answer is:
sire
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">filly's father</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)