Simultaneous utterance (6)
I believe the answer is:
unison
'simultaneous utterance' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for unison that I've seen before include "State of corresponding exactly; occurring together" , "All singing the same notes" , "Harmony - public services' trade union" , "All voices together" , "using same pitch" .)