Female dog (5)
(Other definitions for birch that I've seen before include "Tree, beat" , "Tree with grey or white bark" , "Wood" , "to beat the rest?" , "Hardy tree" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">female dog</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for bitch that I've seen before include "grouse" , "Malicious woman" , "Female dog" , "Female animal" , "Retriever likely to drop" .)