Group of three singers (4)
I believe the answer is:
trio
'group of three singers' 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 "Archduke, possibly" , "Beethoven's 'Archduke', for example" , "Ensemble musical piece, eg Beethoven's 'Archduke'" , "Three of a kind" , "A group or set of three" .)