One rubbish dessert set back a restaurant (9)
I believe the answer is:
trattoria
'restaurant' is the definition.
(I've seen this before)
'one rubbish dessert set back a' is the wordplay.
'one' becomes 'i' (Roman numeral).
'rubbish' becomes 'rot' (I've seen this before).
'dessert' becomes 'tart'.
'set back' says the letters should be written in reverse.
'i'+'rot'+'tart'='irottart'
'irottart' reversed gives 'trattori'.
'trattori'+'a'='TRATTORIA'
(Other definitions for trattoria that I've seen before include "Place to eat" , "Italian eating establishment" , "Italian restaurant" , "where Italians eat" .)