Thin note on one instrument (6)
I believe the answer is:
dilute
'thin' is the definition.
('dilute' can be a synonym of 'thin')
'note on one instrument' is the wordplay.
'note' becomes 'd' (musical note).
'on' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'instrument' becomes 'lute' (lute is a kind of instrument).
'd'+'i'+'lute'='DILUTE'
(Other definitions for dilute that I've seen before include "Add water to another liquid" , "Adulterate" , "Thin down by adding water" , "Make (a liquid) thinner" , "Lessen the strength of a solution" .)