Heard singer's note (6)
I believe the answer is:
'note' is the definition.
(a tenner is a 10 pound note)
'heard singer's' is the wordplay.
'heard' shows a homophone (sound like).
'singer' becomes 'tenor' (tenor is a kind of singer).
'tenor' sounds like 'TENNER'.
(Other definitions for tenner that I've seen before include "PS10" , "sounded by Peter Pears" , "British note" , "Bank note" , "little money" .)