One who is at liberty to be extravagant? (4,7)

I believe the answer is:

free spender

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'one who' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'at liberty to be extravagant?' is the wordplay.
'at liberty' becomes 'free' (similar in meaning).
'to be extravagant?' becomes 'spender' (I can't explain this - if you can you should give a lot more credence to this answer).
'free'+'spender'='FREE SPENDER'

'is' is the link.

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I've seen this clue in The Globe and Mail.

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