Tree of the birch family (5)
I believe the answer is:
'tree of the birch family' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for alder that I've seen before include "One [TYPE OF TREE]" , "A tree for Adler" , "Water resistant wood" , "Catkin-bearing tree" , "Shrub or tree with cone-like fruits" .)