Group of eight singers (5)
I believe the answer is:
octet
'group of eight singers' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for octet that I've seen before include "Set of eight" , "Chamber ensemble, a Mendelssohn favourite" , "Medieval Latin hymn" , "Work for eight players" , "Mendelssohn wrote this" .)