Bill might be ready for a singalong (7)
I believe the answer is:
invoice
'bill might be' is the definition.
(invoice is a type of bill)
I can't explain the rest of the clue.
(Other definitions for invoice that I've seen before include "Document detailing costs - ready to sing (7;2,5)" , "Statement of money owed" , "List of goods sold" , "Statement for goods" , "Send bill for payment" .)