Trim the tree (6)
I believe the answer is:
spruce
I believe this is a double definition.
'trim' is the first definition.
('spruce' can be a synonym of 'trim')
'the tree' is the second definition.
(spruce is a kind of tree)
(Other definitions for spruce that I've seen before include "tree with needle-like leaves" , "ornamental tree" , "Smart, trim" , "perfect condition" , "Neat and smart like pine tree" .)