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' says the letters should be written in reverse.
'tub' backwards 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" .)