Counter; deny (6)
I believe the answer is:
refute
I believe this clue is a double definition.
'counter' is the first definition.
(thesaurus)
'deny' is the second definition.
(I've seen this before)
(Other definitions for refute that I've seen before include "Contradict, prove that statement is wrong" , "Prove something to be wrong" , "Disprove" , "Prove that statement or accusation is wrong" , "Prove to be incorrect" .)