Heard a few notes in harmony (6)
I believe the answer is:
accord
'harmony' is the definition.
(accord is a kind of harmony)
'heard a few notes in' is the wordplay.
'a few notes' can mean three replacements for 'note'.
'heard' becomes 'cor' (I can't justify this - if you can you should give a lot more credence to this answer).
'note' becomes 'a' (musical note).
'note' becomes 'c' (musical note).
'note' becomes 'd' (musical note).
'in' means one lot of letters goes inside another.
'a'+'c'+'d'='acd'
'cor' put into 'acd' is 'ACCORD'.
Can you help me to learn more?
(Other definitions for accord that I've seen before include "Unanimity" , "Peace and agreement" , "Model" , "Harmonize" , "Agree" .)