Three notes, small payments (4)
I believe the answer is:
fees
'payments' is the definition.
(payments for a service)
'three notes small' 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).
'small' becomes 's' (abbreviation - e.g. clothes size).
'f'+'e'+'e'+'s'='FEES'
(Other definitions for fees that I've seen before include "Lawyers' charges" , "Charges, payments" , "Professional payments" , "School payment(s), for example" , "pay" .)