Sanction naughty boy, lip given a brief curl (10)
I believe the answer is:
imprimatur
'sanction' is the definition.
(I know that sanction can be written as imprimatur)
'naughty boy lip given a brief curl' is the wordplay.
'naughty boy' becomes 'imp' (imp can mean a misbehaving child).
'lip' becomes 'rim' (synonyms).
'given' is a charade indicator (letters next to each other).
'a brief curl' becomes 'atur' (I can't explain this - if you can you should give a lot more credence to this answer).
'imp'+'rim'+'atur'='IMPRIMATUR'
Can you help me to learn more?
(Other definitions for imprimatur that I've seen before include "church licence" , "Church approval" .)