Same-note singing (6)
I believe the answer is:
unison
'same-note singing' 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 "Sounding or singing together at the same pitch" , "using same pitch" , "Concordance" , "A coincidence of musical notes" , "All voices together" .)