Adolescents irregularly eat greens (9)
I believe the answer is:
teenagers
'adolescents' is the definition.
('teenager' can be a synonym of 'adolescent')
'irregularly eat greens' is the wordplay.
'irregularly' indicates anagramming the letters (I've seen this in other clues).
'eat'+'greens'='eatgreens'
'eatgreens' is an anagram of 'TEENAGERS'.
(Other definitions for teenagers that I've seen before include "Youngsters" , "They're aged between 13 and 19" , "They're between the onset of puberty and maturity, generally" , "Young people" , "Adolescents" .)