Restaurant where a rubbish dessert served up, one scoffed (9)
I believe the answer is:
trattoria
'restaurant' is the definition.
(type of Italian restaurant)
'a rubbish dessert served up one scoffed' is the wordplay.
'a rubbish' becomes 'rot' (both can mean nonsense).
'dessert' becomes 'tart' (type of dessert).
'served up' is a reversal indicator (in a down clue, going upwards).
'one scoffed' becomes 'ia' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'rot'+'tart'='rottart'
'rottart' reversed gives 'trattor'.
'trattor'+'ia'='TRATTORIA'
'where' acts as a link.
Can you help me to learn more?
(Other definitions for trattoria that I've seen before include "Italian eating establishment" , "where Italians eat" , "Place to eat" , "Italian restaurant" .)