Again, the past rebuilding of Rome (4,4)
I believe the answer is:
once more
'again' is the definition.
(synonyms)
'past rebuilding of rome' is the wordplay.
'past rebuilding' becomes 'once' (I can't explain this - if you can you should believe this answer much more).
'of' indicates anagramming the letters (the answer comes from or 'of' the letters).
'rome' anagrammed gives 'more'.
'once'+'more'='ONCE MORE'
'the' is the link.
Can you help me to learn more?
(Another definition for once more that I've seen is " Again".)