City developer rebuilt our slum (7)
I believe the answer is:
romulus
'city developer' is the definition.
(mythical founder of Rome with his brother Remus)
'rebuilt our slum' is the wordplay.
'rebuilt' is an anagram indicator.
'our'+'slum'='ourslum'
'ourslum' anagrammed gives 'ROMULUS'.
(Other definitions for romulus that I've seen before include "Legendary founder of Rome" , "Twin brother of Remus" , "In legend, one of the founders of Rome" , "Twin founder of city" , "City developer" .)