Beech or birch (4)
I believe the answer is:
tree
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">beech</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >'<span class="explanation-format__original">birch</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for tree that I've seen before include "Chestnut or elm?" , "Plant with trunk (which families have)" , "Plane or pine, for example?" , "Stand for mugs" , "Be up a .... to be in difficulties" .)