Instrument reversing an objection (4)
I believe the answer is:
tuba
'instrument' is the definition.
(tuba is a kind of instrument)
'reversing an objection' is the wordplay.
'reversing' is a reversal indicator.
'an' becomes 'a'.
'objection' becomes 'but' (as in 'no ifs, no buts').
'a'+'but'='abut'
'abut' backwards is 'TUBA'.
(Other definitions for tuba that I've seen before include "bit of brass" , "It's not top brass" , "music-maker" , "member of band" , "Large brass wind instrument" .)