Notes assume a down payment (7)
I believe the answer is:
deposit
'a down payment' is the definition.
('deposit' can be a synonym of 'down payment')
'notes assume' is the wordplay.
'notes' becomes 'de' (two musical notes).
'assume' becomes 'posit' (I've seen this in another clue).
'de'+'posit'='DEPOSIT'
(Other definitions for deposit that I've seen before include "(Money) laid down" , "Sediment or bank lodgment" , "Topside (anag) - put down" , "Precipitate" , "Down payment made to secure goods" .)