Whip left in front of tree (4)
I believe the answer is:
lash
'whip' is the definition.
(synonyms)
'left in front of tree' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in front of' says to put letters next to each other (some letters go in front of others).
'tree' becomes 'ash' (ash is a kind of tree).
'l'+'ash'='LASH'
(Other definitions for lash that I've seen before include "Birch" , "Beat; fasten with cord" , "Flexible strip of a whip" , "Hit with a whip" , "Stroke of the whip" .)