Support rebellious modern teens (11)
I believe the answer is:
endorsement
'support' is the definition.
(I know that endorsement is a type of support)
'rebellious modern teens' is the wordplay.
'rebellious' indicates anagramming the letters.
'modern'+'teens'='modernteens'
'modernteens' anagrammed gives 'ENDORSEMENT'.
(Other definitions for endorsement that I've seen before include "Official sanction or approval" , "Note on a driving licence" , "Penalty appended to driving licence" , "drivers would prefer not to have?" , "Green light" .)