Birch's kin (5)
I believe the answer is:
alder
'birch's 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 "One [TYPE OF TREE]" , "Tree whose bark is used in tanning" , "Artificial fishing fly" , "Type of birch tree" , "Forward movement" .)