Again, the past rebuilding of Rome (4,4)
I believe the answer is:
once more
'again' is the definition.
(synonyms)
'the past rebuilding of rome' is the wordplay.
'the past rebuilding' becomes 'once' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'of' indicates anagramming the letters (the answer comes from or 'of' the letters).
'rome' with letters rearranged gives 'more'.
'once'+'more'='ONCE MORE'
Can you help me to learn more?
(Another definition for once more that I've seen is " Again".)