The actual cask is brought in as disproof (8)
I believe the answer is:
rebuttal
'disproof' is the definition.
(I've seen this before)
'the actual cask is brought in' is the wordplay.
'the actual' becomes 'real' (I've seen this before).
'cask' becomes 'butt' (butt is a kind of cask).
'is brought in' indicates putting letters inside.
'real' enclosing 'butt' is 'REBUTTAL'.
'as' acts as a link.
(Other definitions for rebuttal that I've seen before include "Disproof" , "Answer" , "arguing to the contrary" , "resistance to attack" , "A contradiction of evidence" .)