An under-nine play group (5)
I believe the answer is:
octet
'group' is the definition.
(group of eight musicians)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for octet that I've seen before include "A number of musicians" , "music group" , "Piece for eight players" , "Work for eight players" , "Eightsome" .)