Reversed objection to a brass instrument (4)
I believe the answer is:
tuba
'brass instrument' is the definition.
(I've seen this in another clue)
'reversed objection to a' is the wordplay.
'reversed' is a reversal indicator.
'objection' becomes 'but' (as in 'no ifs, no buts').
'but' backwards is 'tub'.
'tub'+'a'='TUBA'
(Other definitions for tuba that I've seen before include "music-maker" , "Lowest brass wind instrument" , "part of 14 [ORCHESTRA] ?" , "Brass instrument of low pitch" , "Euphonium" .)