Muses, say, why tennis can't be played? (5)
I believe the answer is:
'muses say' is the definition.
The definition suggests a singular noun which matches the answer.
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for nonet that I've seen before include "Tonne (anag.); music for a group" , "Piece for nine musicians" , "Composition for nine performers" , "musical piece" , "Band of nine?" .)