Not a capital expression (13)

I believe the answer is:

provincialism

I'm a little stuck... Click here to teach me more about this clue!

'not a' is the definition.
I can't judge whether this defines the answer.

'capital expression' is the wordplay.
I cannot really see how this works, but
'capital' could be 'provincial' (provincial capital is a kind of capital) and 'provincial' is present in the answer.
The remaining letters 'ism' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in The Irish Independent.

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