Tidy tree aloft (6,2)
I believe the answer is:
spruce up
'tidy' is the definition.
(thesaurus)
'tree aloft' is the wordplay.
'tree' becomes 'spruce' (spruce is a kind of tree).
'aloft' becomes 'up' (up above).
'spruce'+'up'='SPRUCE UP'
(Other definitions for spruce up that I've seen before include "Become smart" , "Titivate" , "Make smarter" , "groom" .)