Youth ate green assortment (8)
I believe the answer is:
teenager
'youth' is the definition.
(I've seen this before)
'ate green assortment' is the wordplay.
'assortment' indicates an anagram.
'ate'+'green'='ategreen'
'ategreen' anagrammed gives 'TEENAGER'.
(Other definitions for teenager that I've seen before include "14, perhaps" , "one that's not yet made a score?" , "Young person" , "Youngster" , "Adolescent" .)