Objection to returning tub (3)
I believe the answer is:
but
'objection' is the definition.
(as in 'no ifs, no buts')
'returning tub' is the wordplay.
'returning' is a reversal indicator.
'tub' written backwards gives 'BUT'.
'to' is the link.
(Other definitions for but that I've seen before include "Conjunction meaningn on the other hand" , "On the contrary" , "However" , "Only; apart from" , "Save" .)