Current books on pastry standing in restaurant (9)
I believe the answer is:
'restaurant' is the definition.
(type of Italian restaurant)
'current books on pastry standing' is the wordplay.
'current' becomes 'air' (a current of air).
'books' becomes 'OT' (Old Testament).
'on' says to put letters next to each other.
'pastry' becomes 'tart' (tart is a kind of pastry).
'standing' shows that the letters should be reversed in order (in a down clue, letters stand up).
'airottart' back-to-front is 'TRATTORIA'.
'in' acts as a link.
(Other definitions for trattoria that I've seen before include "Italian restaurant", "where Italians eat", "Place to eat", "Italian eating establishment".)