Duo plus one (4)
I believe the answer is:
trio
'duo plus one' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for trio that I've seen before include "Ensemble musical piece, eg Beethoven's 'Archduke'" , "Piece for three players" , "Archduke, possibly" , "wise men?" , "Small group playing" .)