Cat left tree (4)
I believe the answer is:
lash
'cat' is the definition.
(I've seen this before)
'left tree' is the wordplay.
'left' becomes 'l' (common abbreviation).
'tree' becomes 'ash' (ash is a kind of tree).
'l'+'ash'='LASH'
(Other definitions for lash that I've seen before include "Fasten with rope; whip" , "Stroke of the whip" , "Sweep" , "Whip or scourge" , "Flog" .)