Singer's sound note (6)
I believe the answer is:
tenner
'note' is the definition.
(a tenner is a 10 pound note)
'singer's sound' is the wordplay.
'singer' becomes 'tenor' (tenor is a kind of singer).
'sound' shows a homophone (sound like).
'tenor' is a homophone of 'TENNER'.
(Other definitions for tenner that I've seen before include "Banknote (informal); sounds like a singer" , "A 10 pound note (colloq.)" , "British note" , "UK banknote" , "note form" .)