Auditor’s treat at open air restaurant (9)
I believe the answer is:
cafeteria
'restaurant' is the definition.
(cafeteria is a kind of restaurant)
'auditor's treat at open air' is the wordplay.
I cannot really understand how this works, but
an anagram of 'air' is 'ria' which is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for cafeteria that I've seen before include "Place to eat" , "Self-service restaurant" , "dining-room" , "place of refreshment" , "where food is served" .)