Three-note chords (6)
I believe the answer is:
triads
'three-note chords' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for triads that I've seen before include "Threesomes" , "Sets of threes" , "Groups of sets of three" , "Chinese secret societies involved in crime" , "Chinese criminal gangs" .)