Singer heard producing a note (6)
I believe the answer is:
tenner
'a note' is the definition.
(a tenner is a 10 pound note)
'singer heard' is the wordplay.
'singer' becomes 'tenor' (tenor is a kind of singer).
'heard' shows a homophone (sound like).
'tenor' sounds like 'TENNER'.
'producing' is the link.
(Other definitions for tenner that I've seen before include "Monetary note" , "UK banknote" , "Bank note" , "Two fivers" , "Cash note" .)