Impotent Rome a dilapidated superpower now gone (7,6)
I believe the answer is:
ottoman empire
'superpower now gone' is the definition.
(I've seen this before)
'impotent rome a dilapidated' is the wordplay.
'dilapidated' is an anagram indicator.
'impotent'+'rome'+'a'='impotentromea'
'impotentromea' is an anagram of 'OTTOMAN EMPIRE'.
(Other definitions for ottoman empire that I've seen before include "superpower now gone" , "Turkish dominion" , "Former Turkish sultanate" .)