Affirm positively (10)
I believe the answer is:
asseverate
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">affirm positively</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for asseverate that I've seen before include "Solemnly declare" , "Claim" , "State categorically" , "Solemnly say" , "Swear" .)