Young female horse (5)
I believe the answer is:
filly
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">young female horse</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for filly that I've seen before include "Young female horse" , "Young mare" , "Female horse under four years old" .)