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