Brussels set him upon a bit of brass (9)
I believe the answer is:
euphonium
'a bit of brass' is the definition.
'euphonium' can be an answer for 'brass' (euphonium is a kind of brass). I am not certain of the 'a bit of' bit.
'brussels set him upon' is the wordplay.
'brussels' becomes 'EU' (European Parliament is based in Brussels).
'set' is an anagram indicator (instruction to set the letters).
'him'+'upon'='himupon'
'himupon' with letters rearranged gives 'phonium'.
'eu'+'phonium'='EUPHONIUM'
Can you help me to learn more?
(Other definitions for euphonium that I've seen before include "Tuba-like instrument" , "Wind instrument" , "Bass-tenor tuba" , "brass piece" , "Band member" .)