Disprove conclusions in either case, nevertheless (5)
I believe the answer is:
rebut
'disprove' is the definition.
(rebutting is a kind of disproving)
'conclusions in either case nevertheless' is the wordplay.
'conclusions in' says to take the final letters.
'nevertheless' becomes 'but' (I've seen this before).
The last letters of 'either case' are 're'.
're'+'but'='REBUT'
(Other definitions for rebut that I've seen before include "Refute - disprove" , "Deny claim" , "Contradict or disprove" , "Refure or disprove" , "Rebuff; deny" .)