Group invested in accessory for trumpet or bagpipe (7)
I believe the answer is:
musette
'bagpipe' is the definition.
(musette is a kind of bagpipe)
'group invested in accessory for trumpet' is the wordplay.
'group' becomes 'set' (both can mean a collection).
'invested in' means one lot of letters goes inside another.
'accessory for trumpet' becomes 'mute' (I can't explain this - if you can you should believe this answer much more).
'set' inserted into 'mute' is 'MUSETTE'.
'or' acts as a link.
Can you help me to learn more?
(Other definitions for musette that I've seen before include "Rustic instrument" , "bagpipe" , "Old wind instrument" , "dance" , "Bag" .)