Bath's objection overturned (3)
I believe the answer is:
tub
'bath's' is the definition.
('tub' can be a synonym of 'bath')
'objection overturned' is the wordplay.
'objection' becomes 'but'.
'overturned' says the letters should be written in reverse.
'but' in reverse letter order is 'TUB'.
(Other definitions for tub that I've seen before include "Wooden bath or slow boat" , "Wide bucket-like vessel" , "Small cask" , "Carton" , "Large plant container" .)