Bill ready to sing? (7)
I believe the answer is:
invoice
'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" .)