Refutation afterwards raised to cover objection (8)
I believe the answer is:
rebuttal
'refutation' is the definition.
(I know that rebuttal is a type of refutation)
'afterwards raised to cover objection' is the wordplay.
'afterwards' becomes 'later' (synonyms).
'raised' says the letters should be written in reverse (going up in a down clue).
'to cover' is an insertion indicator.
'objection' becomes 'but' (as in 'no ifs, no buts').
'later' in reverse letter order is 'retal'.
'retal' enclosing 'but' is 'REBUTTAL'.
(Other definitions for rebuttal that I've seen before include "Refuting argument" , "reply to accusation" , "Negation" , "Denial" , "A contradiction of evidence" .)