Dear God, it's an ancient city (5)
I believe the answer is:
petra
'it's an ancient city' is the definition.
(I've seen this before)
'dear god' is the wordplay.
'dear' becomes 'pet' (I've seen this in other clues).
'god' becomes 'ra' (Egyptian god).
'pet'+'ra'='PETRA'
(Other definitions for petra that I've seen before include "Rock-hewn city of Jordan" , "Historic city in Jordan" , "Long-lost city of Jordan rediscovered in 1812" , "Jordan's rock-hewn city" , "Burgon's rose-red city" .)