Disproof in due course turning round objection (8)
I believe the answer is:
rebuttal
'disproof' is the definition.
(I've seen this before)
'in due course turning round objection' is the wordplay.
'in due course' becomes 'later' (I've seen this before).
'turning' says the letters should be written in reverse.
'round' is an insertion indicator.
'objection' becomes 'but'.
'later' backwards is 'retal'.
'retal' going around 'but' is 'REBUTTAL'.
(Other definitions for rebuttal that I've seen before include "statement delivered to counter" , "reply to accusation" , "Counter-argument" , "A disproving or rejection of evidence" , "Denial" .)