Diner may roughly celebrate uplifting atmosphere (9)
I believe the answer is:
cafeteria
'diner' is the definition.
(thesaurus)
'roughly celebrate uplifting atmosphere' is the wordplay.
'roughly' becomes 'ca' (abbreviation for circa).
'celebrate' becomes 'fete' (synonyms).
'uplifting' says the letters should be written backwards.
'atmosphere' becomes 'air' (synonyms).
'air' backwards is 'ria'.
'ca'+'fete'+'ria'='CAFETERIA'
'may' is the link.
(Other definitions for cafeteria that I've seen before include "dining-room" , "Place to eat" , "Where one scoffs" , "eating-place" , "cheap restaurant" .)