One rubbish bitter served up by a bistro (9)
I believe the answer is:
trattoria
'bistro' is the definition.
(thesaurus)
'one rubbish bitter served up by a' is the wordplay.
'one rubbish' becomes 'rot' (both can mean nonsense).
'bitter' becomes 'tart' (tart can mean bitter-tasting).
'served up' is a reversal indicator (in a down clue, going upwards).
'by a' becomes 'ia' (I can't explain this - if you can you should give a lot more credence to this answer).
'rot'+'tart'='rottart'
'rottart' back-to-front is 'trattor'.
'trattor'+'ia'='TRATTORIA'
Can you help me to learn more?
(Other definitions for trattoria that I've seen before include "Place to eat" , "Italian eating establishment" , "where Italians eat" , "Italian restaurant" .)