Birch kin (5)
I believe the answer is:
alder
'birch kin' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for alder that I've seen before include "Tree akin to the birch" , "Birch relative" , "New Forest resident" , "Forward movement" , "A tree for Adler" .)