Adolescent ate green bananas
I believe the answer is:
teen-ager
'adolescent' is the definition.
('teen-ager' can be a synonym of 'adolescent')
'ate green bananas' is the wordplay.
'bananas' indicates an anagram (informal term for mad).
'ate'+'green'='ategreen'
'ategreen' is an anagram of 'TEEN-AGER'.
(Other definitions for teen-ager that I've seen before include "One between 12 and 20 years old, exclusive" , "Green tea for youngster" , "Green tea for young person" , "Green tea (anag.)" , "One between twelve and twenty" .)