Objection to return a bit of brass (4)
I believe the answer is:
tuba
'bit of brass' is the definition.
(I've seen this before)
'objection to return a' is the wordplay.
'objection' becomes 'but'.
'to return' says the letters should be written backwards.
'but' backwards is 'tub'.
'tub'+'a'='TUBA'
(Other definitions for tuba that I've seen before include "blow it!" , "Brass instrument of low pitch" , "Producer of note" , "One's blown" , "horn" .)