Whip large tree (4)
I believe the answer is:
lash
'whip' is the definition.
(synonyms)
'large tree' is the wordplay.
'large' becomes 'l'.
'tree' becomes 'ash' (ash tree is a kind of tree).
'l'+'ash'='LASH'
(Other definitions for lash that I've seen before include "Flexible strip of a whip" , "Flog" , "Bind tightly with a rope or cord" , "Secure with rope" , "Hit with a whip" .)