Oscar classifies trees (7)
I believe the answer is:
oranges
'trees' is the definition.
(orange is a kind of tree)
'oscar classifies' is the wordplay.
'oscar' becomes 'o' (phonetic alphabet: alpha, bravo, charlie etc.).
'classifies' becomes 'ranges' (I've seen this before).
'o'+'ranges'='ORANGES'
(Other definitions for oranges that I've seen before include "a zesty lot" , "Go snare these fruits" , "See 8" , "Citrus fruits" , "Mandarins, satsumas etc." .)