Decline flattens out finances (9)

I believe the answer is:

bankrolls

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'finances' is the definition.

'decline flattens out' is the wordplay.
'decline' becomes 'bank' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'flattens out' becomes 'rolls'.
'bank'+'rolls'='BANKROLLS'

Can you help me to learn more?

(Another definition for bankrolls that I've seen is " Finances, a business say".)

I've seen this clue in The Guardian.

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