Dispute the truth of (6)
I believe the answer is:
impugn
'dispute the truth of' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for impugn that I've seen before include "Challenge (e.g. honesty)" , "Express doubts about truth or honesty of, say motives" , "Try to discredit, express doubts about honesty" , "Attack verbally" , "Attack as false, challenge" .)