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