Eatery originally conveyed a festival atmosphere, on reflection (9)
I believe the answer is:
cafeteria
'eatery' is the definition.
(thesaurus)
'originally conveyed a festival atmosphere on reflection' is the wordplay.
'originally' indicates taking the first letters.
'festival' becomes 'fete' ('fete' can be a synonym of 'festival'**).
'atmosphere' becomes 'air' ('air' can be a synonym of 'atmosphere').
'on reflection' is a reversal indicator.
The initial letter of 'conveyed' is 'c'.
'air' backwards is 'ria'.
'c'+'a'+'fete'+'ria'='CAFETERIA'
(Other definitions for cafeteria that I've seen before include "greasy spoon?" , "dining-room" , "eating-place" , "Self-help facility" , "Where one scoffs" .)