Superb vehicle overturned by tree (7)
I believe the answer is:
sublime
'superb' is the definition.
(thesaurus)
'vehicle overturned by tree' is the wordplay.
'vehicle' becomes 'bus' (I've seen this before).
'overturned' says the letters should be written backwards.
'by' says to put letters next to each other.
'tree' becomes 'lime' (lime is a kind of tree**).
'bus' backwards is 'sub'.
'sub'+'lime'='SUBLIME'
(Other definitions for sublime that I've seen before include "Exalted, lofty" , "Transcendent" , "Awe-inspiring - exalted" , "Heavenly" , "Of great excellence or beauty, awe-inspiring" .)