Trees in the birch family (6)
I believe the answer is:
alders
'trees in the birch family' is the definition.
(I've seen this in another clue)
This is the entire clue.
(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" .)