Bill ready to sing? (7)

I believe the answer is:

invoice

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'bill' is the definition.
(I know that bill can be written as invoice)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for invoice that I've seen before include "ask for payment" , "Bill (fit to sing?)" , "Give account" , "Demand for payment" , "Demand for sum due" .)

I've seen this clue in the Sydney Morning Herald.

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