The common people march about, full of spirit (9)
I believe the answer is:
democracy
'the common people' is the definition.
The answer and definition can be both groups as well as being singular nouns.
Maybe there's an association between them I don't understand?
'march about full of spirit' is the wordplay.
'march' becomes 'demo' (as in a demonstration).
'about' becomes 'c' (abbreviation for circa).
'full of spirit' becomes 'racy' (I can't explain this - if you can you should give a lot more credence to this answer).
'demo'+'c'+'racy'='DEMOCRACY'
Can you help me to learn more?
(Other definitions for democracy that I've seen before include "Government of/by/for all the people" , "The least worst political system?" , "expression of popular will?" , "form of government" .)