Returned an objection to an instrument (4)
I believe the answer is:
tuba
'instrument' is the definition.
(tuba is a kind of instrument)
'returned an objection to an' is the wordplay.
'returned' says the letters should be written in reverse.
'an objection' becomes 'but'.
'to an' becomes 'a'.
'but' backwards is 'tub'.
'tub'+'a'='TUBA'
(Other definitions for tuba that I've seen before include "Producer of note" , "item band might play" , "A bass instrument" , "Big brass instrument" , "Large brass wind instrument" .)