Tree related to the birch (5)
I believe the answer is:
alder
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">tree related to the birch</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 alder that I've seen before include "Native tree" , "Tree whose bark is used in tanning" , "Tree akin to the birch" , "Type of birch tree" , "Forward movement" .)