Youngster ate green bananas (8)
I believe the answer is:
teenager
'youngster' is the definition.
(I know that teenager is a type of youngster)
'ate green bananas' is the wordplay.
'bananas' indicates anagramming the letters (bananas can informally mean crazy).
'ate'+'green'='ategreen'
'ategreen' anagrammed gives 'TEENAGER'.
(Other definitions for teenager that I've seen before include "Young person" , "14, perhaps" , "Old child or young adult?" , "one with an unpredictable nature" , "Generate (anag)" .)