Singers heard producing notes (7)
I believe the answer is:
tenners
'notes' is the definition.
(ten pound notes)
'singers heard' is the wordplay.
'singers' becomes 'tenors' (tenor is a singing voice).
'heard' shows a homophone (sound like).
'tenors' sounds like 'TENNERS'.
'producing' is the link.
(Other definitions for tenners that I've seen before include "PS10 notes" , "Monetary notes" .)