Objection to changing tub (3)
I believe the answer is:
but
'objection' is the definition.
(as in 'no ifs, no buts')
'changing tub' is the wordplay.
'changing' indicates an anagram.
'tub' is an anagram of 'BUT'.
'to' acts as a link.
(Other definitions for but that I've seen before include "Only in attribution" , "Conjunction meaningn on the other hand" , "On the contrary" , "However" , "Save" .)