I'm Pope, I rebuilt an old Roman city (7)
I believe the answer is:
pompeii
'an old roman city' is the definition.
'i'm pope i rebuilt' is the wordplay.
'rebuilt' is an anagram indicator.
'im'+'pope'+'i'='impopei'
'impopei' with letters rearranged gives 'POMPEII'.
(Other definitions for pompeii that I've seen before include "Roman site" , "Ancient city buried under Mount Vesuvius ash" , "Old Italian city" , "Former city" , "Italian town near Naples buried by Vesuvius in AD 79" .)