Ancient city in display's a revelation (8)
I believe the answer is:
exposure
'a revelation' is the definition.
(I know that exposure is a type of revelation)
'ancient city in display's' is the wordplay.
'ancient city' becomes 'ur' (ancient Sumerian city state).
'in' means one lot of letters goes inside another.
'display' becomes 'expose' (synonyms).
'ur' going into 'expose' is 'EXPOSURE'.
(Other definitions for exposure that I've seen before include "Publicity, or state of being revealed" , "Appearance before the public" , "being unmasked" , "Use of flash" , "Lack of shelter" .)