Duet plus one (4)
I believe the answer is:
trio
'duet plus one' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for trio that I've seen before include "Three of a kind" , "Beethoven's 'Archduke', for example" , "Beethoven's 'Archduke': famous piece written for this ensemble" , "Work for three" , "Chamber group" .)