Young people unhappily eat greens (9)
I believe the answer is:
teenagers
'young people' is the definition.
(people aged between twelve and twenty)
'unhappily eat greens' is the wordplay.
'unhappily' indicates an anagram.
'eat'+'greens'='eatgreens'
'eatgreens' with letters rearranged gives 'TEENAGERS'.
(Other definitions for teenagers that I've seen before include "Young people" , "They're between the onset of puberty and maturity, generally" , "They're aged between 13 and 19" , "Youngsters" , "Adolescents" .)