Youths (4)
'youths' is the definition.
(lad means a boy or young man)
This is the entire clue.
(Other definitions for lads that I've seen before include "Boys, young fellows" , "Fellows, blokes" , "Young men" , "Youngsters" , "Fellows, chaps" .)
(Other definitions for teenagers that I've seen before include "Young people" , "They're aged between 13 and 19" , "Adolescents" , "Youngsters" , "They're between the onset of puberty and maturity, generally" .)