Smart tree (6)
I believe the answer is:
spruce
I believe this clue is a double definition.
'smart' is the first definition.
(synonyms)
'tree' is the second definition.
(spruce is a kind of tree)
(Other definitions for spruce that I've seen before include "ornamental tree" , "wood" , "thing for fairy to be on top of?" , "Trim - conifer" , "tree with needle-like leaves" .)