Where Dickens and Darwin once for example found a voice to be heard (6)
I believe the answer is:
tenner
'where dickens and darwin once for example found' is the definition.
(Dickens and Darwin appeared on ten pound notes)
'a voice to be heard' is the wordplay.
'a voice' becomes 'tenor' (tenor is a kind of voice).
'to be heard' shows a homophone (sound like).
'tenor' is a homophone of 'TENNER'.
(Other definitions for tenner that I've seen before include "Bank note" , "Heard bell" , "British note" , "Banknote (informal); sounds like a singer" , "Note - sounds like singer" .)