Bistro loses new dinner (5)
I believe the answer is:
diner
'bistro' is the definition.
(thesaurus)
'loses new dinner' is the wordplay.
'loses' suggests deleting specific letters.
'new' becomes 'n'.
'dinner' with 'n' taken away is 'DINER'.
(Other definitions for diner that I've seen before include "American restaurant" , "(Person eating in) restaurant" , "one taking several courses" , "(One using) restaurant car" , "Restaurant client" .)