Plunder with the exception of an old piece of brass (7)
I believe the answer is:
sackbut
'brass' is the definition.
(I know that sackbut is a type of trombone)
'plunder with the exception of an old piece' is the wordplay.
'plunder' becomes 'sack' (synonyms).
'with' says to put letters next to each other.
'the exception of an old piece' becomes 'but' (I can't justify this - if you can you should give a lot more credence to this answer).
'sack'+'but'='SACKBUT'
'of' is the link.
Can you help me to learn more?
(Other definitions for sackbut that I've seen before include "music maker" , "antique instrument" , "Ancient musical instrument" , "Medieval form of trombone" , "one on the slide" .)