Tree related to the birch (5)
I believe the answer is:
alder
'tree related to the birch' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for alder that I've seen before include "Native tree" , "Tree whose bark is used in tanning" , "Tree akin to the birch" , "Type of birch tree" , "Forward movement" .)