Nonets written without piano, including a coda from brass (7)
I believe the answer is:
enneads
'nonets' is the definition.
(I've seen this in another clue)
'written without piano including a coda from brass' is the wordplay.
'written' becomes 'penned' (written with a pen).
'without' is a deletion indicator.
'piano' becomes 'p' (musical abbreviation).
'including' means one lot of letters goes inside another.
'coda from brass' becomes 's' (I can't explain this - if you can you should give a lot more credence to this answer).
'penned' with 'p' removed is 'enned'.
'enned' enclosing 'a' is 'ennead'.
'ennead'+'s'='ENNEADS'
Can you help me to learn more?
(Other definitions for enneads that I've seen before include "Eights plus coxes, perhaps" , "Sane end (anag.)" , "groups of nine" , "Groups with members one over the eight" , "Groups appearing one over the eight" .)