Be adamant (6)
I believe the answer is:
insist
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">be adamant</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 insist that I've seen before include "express convinced viewpoint" , "Be firmly demanding" , "Be adamant" , "Declare or demand emphatically" , "Take a firm stand" .)