Serving up puff pastry cases closed restaurant (9)
I believe the answer is:
trattoria
'serving' is the definition.
Both the answer and definition are singular nouns.
Maybe there's an association between them I don't understand?
'up puff pastry cases closed restaurant' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'puff' becomes 'air'.
'pastry' becomes 'tart' (tart is a kind of pastry).
'cases' indicates putting letters inside.
'closed restaurant' becomes 'ot' (I can't justify this - if you can you should give a lot more credence to this answer).
'air'+'tart'='airtart'
'airtart' going around 'ot' is 'airottart'.
'airottart' reversed gives 'TRATTORIA'.
(Other definitions for trattoria that I've seen before include "where Italians eat" , "Italian eating establishment" , "Place to eat" , "Italian restaurant" .)