Charge three notes (3)
I believe the answer is:
fee
'charge' is the definition.
(fee is a kind of charge)
'three notes' is the wordplay.
'three notes' can mean three replacements for 'note'.
'note' becomes 'f' (musical note).
'note' becomes 'e' (musical note).
'note' becomes 'e' (musical note).
'f'+'e'+'e'='FEE'
(Other definitions for fee that I've seen before include "Amount paid for job" , "Cost" , "(Entry) charge" , "Sum payable to, say, barrister" , "Charge for professional services" .)