Old city I'd help to rebuild (6)
I believe the answer is:
delphi
'old city' is the definition.
(I've seen this before)
'i'd help to rebuild' is the wordplay.
'to rebuild' indicates anagramming the letters (I've seen 'rebuilt' mean this).
'id'+'help'='idhelp'
'idhelp' with letters rearranged gives 'DELPHI'.
(Other definitions for delphi that I've seen before include "'Ancient Greek city, site of the oracle (6)'" , "Old programming language" , "'In Greek mythology, the site of the oracle (6)'" , "Where Apollo's most famous oracle spoke in Greece" , "Scene of ancient oracle" .)