Group of players mostly penning quiet pieces for instruments (9)
I believe the answer is:
equipment
'instruments' is the definition.
(both can mean tools or devices)
'group of players mostly penning quiet pieces' is the wordplay.
'group of players' becomes 'Equity' (UK trade union including actors).
'mostly' means to remove the last letter.
'penning' indicates putting letters inside.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'pieces' becomes 'men' (man can mean a chess piece).
'equity' with its last letter taken away is 'equit'.
'p'+'men'='pmen'
'equit' enclosing 'pmen' is 'EQUIPMENT'.
'for' is the link.
(Other definitions for equipment that I've seen before include "outfit" , "Apparatus, tools" , "fitting" , "Set of instruments that perform a service" , "Kit, apparatus" .)