Complete agreement (6)
I believe the answer is:
unison
'complete agreement' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for unison that I've seen before include "All acting the same way at the same time" , "Concord; a trade union" , "All singing the same notes" , "Total agreement or concord" , "Concord, sounding or singing together" .)