Simultaneous utterance (6)
I believe the answer is:
unison
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">simultaneous utterance</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)