Cavort with traitor at restaurant. (9)
I believe the answer is:
trattoria
'restaurant' is the definition.
(type of Italian restaurant)
'cavort with traitor at' is the wordplay.
'cavort' indicates anagramming the letters (I've seen 'cavorting' mean this).
'with' means one lot of letters go next to another.
'traitor' next to 'at' is 'traitorat'.
'traitorat' with letters rearranged gives 'TRATTORIA'.
(Other definitions for trattoria that I've seen before include "where Italians eat" , "Place to eat" , "Italian eating establishment" , "Italian restaurant" .)