Trees in the birch family (6)
I believe the answer is:
alders
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">trees in the birch family</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for alders that I've seen before include "Catkin-bearing trees" , "Trees for red Sal" , "Trees with toothed leaves" , "Trees of the birch family" , "Al's red trees, strangely" .)