Simultaneous action (6)
I believe the answer is:
unison
'simultaneous action' 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 "Simultaneous utterance" , "Concord; with same pitch" , "Sameness in pitch" , "A coincidence of musical notes" , "State of corresponding exactly; occurring together" .)