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