Asseverate (6)
I believe the answer is:
affirm
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">asseverate</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 affirm that I've seen before include "Publicly state" , "Declare one's commitment (to)" , "State positively" , "Confidently assert" , "Make a formal declaration (rather than an oath)" .)