Group of notes in harmony (5)
I believe the answer is:
chord
'group of notes in harmony' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for chord that I've seen before include "line for mathematicians" , "Notes struck together" , "Combination of notes sounded in harmony" , "Group of notes sounded together" , "Number of notes played together" .)