Accord; harmony (6)
I believe the answer is:
unison
This clue is a double definition.
'accord' is the first definition.
(unison is a kind of accord)
'harmony' is the second definition.
(unison is a kind of harmony)
(Other definitions for unison that I've seen before include "Concord; with same pitch" , "all players directed to the same pitch" , "All act as one in this" , "Trade union (where all agree?)" , "All singing the same note" .)