Female horse (4)
I believe the answer is:
mare
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">female horse</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for mare that I've seen before include "For classicists, sea" , "Nag" , "Ream (anag)" , "Adult female zebra" , "Female quadruped" .)