Adolescents sent off around the back (5)
I believe the answer is:
teens
'adolescents' is the definition.
(I've seen this in another clue)
'sent off around the back' is the wordplay.
'off' indicates anagramming the letters.
'around' is an insertion indicator.
'back' says to take the final letters (the letter at the back).
The last letter of 'the' is 'e'.
'sent' is an anagram of 'tens'.
'tens' placed around 'e' is 'TEENS'.
(Other definitions for teens that I've seen before include "children" , "Adolescence" , "seven-year period" , "Years 13-19" , "13-19 age group" .)