Eg, alder 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">eg alder</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(alder is a kind of tree)</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 "Oak, for example" , "Large growth" , "Forest plant" , "Large, woody perennial plant" , "Eg, elm or oak" .)