Mutter vaguely about piano and brass instrument (7)
I believe the answer is:
trumpet
'brass instrument' is the definition.
(I've seen this before)
'mutter vaguely about piano' is the wordplay.
'vaguely' indicates anagramming the letters.
'about' means one lot of letters goes inside another.
'piano' becomes 'p'.
'mutter' anagrammed gives 'trumet'.
'trumet' placed around 'p' is 'TRUMPET'.
'and' acts as a link.
(Other definitions for trumpet that I've seen before include "Proclaim loudly" , "beastly noise" , "Play it" , "Brass wind instrument" , "Daffodil corona" .)