Large tree, birch (4)
I believe the answer is:
lash
'birch' is the definition.
(I know that birch is a kind of lash)
'large tree' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'tree' becomes 'ash' (ash is a kind of tree).
'l'+'ash'='LASH'
(Other definitions for lash that I've seen before include "Flog" , "Secure with rope" , "Whip or scourge" , "Whip; securely fasten" , "Bind tightly with a rope or cord" .)