Quartet doubled (5)
I believe the answer is:
octet
'quartet doubled' 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 "Eightsome" , "Group of eight players" , "Mendelssohn or Schubert piece" , "Musical group" , "Musical composition for eight performers" .)