Mature filly (4)
I believe the answer is:
mare
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mature filly</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for mare that I've seen before include "Adult female zebra" , "Ream (anag)" , "Nag" , "Horse; lunar sea" , "Female equine animal" .)