Two notes on string producing harmony (6)
I believe the answer is:
accord
'harmony' is the definition.
(accord is a kind of harmony)
'two notes on string' is the wordplay.
'two notes' means two substitutions for 'note'.
'note' becomes 'a' (musical note).
'note' becomes 'c' (musical note).
'on' means one lot of letters go next to another.
'string' becomes 'cord' (I've seen this before).
'a'+'c'+'cord'='ACCORD'
'producing' is the link.
(Other definitions for accord that I've seen before include "binding agreement" , "Official agreement; harmony" , "Peace and agreement" , "Agreement - conformity" , "Harmony, concurrence" .)