Birch relative (5)
I believe the answer is:
alder
'birch relative' 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 "Birch relative" , "Tree with toothed leaves and cone-like fruits" , "Forward movement" , "Tree with catkins" , "Tree akin to the birch" .)