Dispute the truth of (6)
I believe the answer is:
impugn
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">dispute the truth of</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)