Restaurant offers excellent crumble, sweet course served retro-style (9)
I believe the answer is:
trattoria
'restaurant' is the definition.
(type of Italian restaurant)
'excellent crumble sweet course served retro-style' is the wordplay.
'excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'crumble' becomes 'rot' (I've seen this before).
'sweet course' becomes 'tart' (type of pudding or sweet).
'served retro-style' says the letters should be written in reverse (retro means \'back\' in Latin).
'ai'+'rot'+'tart'='airottart'
'airottart' back-to-front is 'TRATTORIA'.
'offers' acts as a link.
(Other definitions for trattoria that I've seen before include "where Italians eat" , "Italian restaurant" , "Italian eating establishment" , "Place to eat" .)