Timeless metropolitan rhyme about former European power (4,5,6)
I believe the answer is:
holy roman empire
'timeless metropolitan rhyme' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'former european power' is the wordplay.
I cannot quite see how this works, but
'european' could be 'roman' (Roman is an example) and 'roman' is found in the answer.
'power' could be 'empire' (I've seen this before) and 'empire' is found in the answer.
The remaining letters 'holy' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'about' acts as a link.
Can you help me to learn more?
(Other definitions for holy roman empire that I've seen before include "unification of Europe" , "earlier power?" , "Charlemagne ruled it first" .)