Stallion's mate (4)
I believe the answer is:
mare
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">stallion's mate</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 mare that I've seen before include "Adult female zebra" , "Adult female horse" , "Nag" , "Female equine animal" , "Horse; lunar sea" .)