Youngsters seen playing behind junction (5)
I believe the answer is:
teens
'youngsters' is the definition.
(I've seen this in another clue)
'seen playing behind junction' is the wordplay.
'playing' indicates an anagram.
'behind' says to put letters next to each other.
'junction' becomes 't' ().
'seen' anagrammed gives 'eens'.
'eens' put after 't' is 'TEENS'.
(Other definitions for teens that I've seen before include "children" , "Adolescent years" , "The years 13 to 19" , "Years 13-19" , "13-19 age group" .)