Notes in harmony (6)
I believe the answer is:
chords
'notes in harmony' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for chords that I've seen before include "Combinations of notes sounded together" , "Groups of notes" , "Combinations of harmonious notes" , "Harmonious blends of notes" , "Combinations of three or more musical notes" .)