Examiner overturning top mark in halls (9)
I believe the answer is:
auditoria
'halls' is the definition.
(I've seen this before)
'examiner overturning top mark' is the wordplay.
'examiner' becomes 'auditor' (an auditor examines accounts etc).
'overturning' says the letters should be written in reverse.
'top mark' becomes 'ai' (resembles A1, which can mean first class).
'ai' written backwards gives 'ia'.
'auditor'+'ia'='AUDITORIA'
'in' acts as a link.
(Other definitions for auditoria that I've seen before include "Eg, concert halls" , "Where concertgoers sit" , "Locations for houses" , "Theatre areas for spectators or listeners" , "Spaces for audiences" .)