Harmony or agreement (6)
I believe the answer is:
unison
This clue is a double definition.
'harmony' is the first definition.
(I know that unison is a type of accord)
'agreement' is the second definition.
(I know that unison is a type of agreement)
(Other definitions for unison that I've seen before include "There's agreement" , "Sameness in pitch" , "All singing the same notes" , "Concordance" , "Simultaneous utterance" .)