Nine gross only? (5)
I believe the answer is:
nonet
'nine' is the definition.
(group of nine musicians)
'gross only?' is the wordplay.
I cannot really see how this works, but
'only?' could be 'one' ('one' can be a synonym of 'only') and 'one' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for nonet that I've seen before include "Group of nine musicians" , "Ennead" , "piece to be performed" , "Music for nine instruments or voices" , "Multi-figure composition" .)