Trio with a tart cooked in restaurant (9)
I believe the answer is:
trattoria
'restaurant' is the definition.
(type of Italian restaurant)
'trio with a tart cooked' is the wordplay.
'with' says to put letters next to each other.
'cooked' is an anagram indicator (letters cooked into a new form).
'a'+'tart'='atart'
'trio' after 'atart' is 'atarttrio'.
'atarttrio' with letters rearranged gives 'TRATTORIA'.
'in' is the link.
(Other definitions for trattoria that I've seen before include "Place to eat" , "where Italians eat" , "Italian restaurant" , "Italian eating establishment" .)