Bill that’s popular, say (7)
I believe the answer is:
invoice
'bill' is the definition.
('invoice' can be a synonym of 'bill')
'popular say' is the wordplay.
'popular' becomes 'in'.
'say' becomes 'voice' (voicing is a kind of saying**).
'in'+'voice'='INVOICE'
'that's' is the link.
(Other definitions for invoice that I've seen before include "Statement of money owed" , "Statement of sum due" , "Statement of account" , "ask for payment" , "Demand for sum due" .)