Eightsome (5)
'eightsome' is the definition.
(I know that eightsome can be written as octet)
This is the entire clue.
(Other definitions for octet that I've seen before include "Played" , "Musical composition for eight performers" , "Lines of verse" , "Group of eight musicians" , "Mendelssohn wrote this" .)
(Other definitions for octad that I've seen before include "Crew, perhaps" , "Group" , "E.g. quarterfinalists" , "quarter-finalists, perhaps" .)