Objection to returning tub (3)
I believe the answer is:
but
'objection' is the definition.
'returning tub' is the wordplay.
'returning' says the letters should be written in reverse.
'tub' back-to-front is 'BUT'.
'to' acts as a 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" .)