Maintain firmly (6)
I believe the answer is:
insist
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">maintain firmly</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 "Demand determinedly" , "Continue to assert, maintain" , "Demand persistently" , "Maintain persistently" , "Be adamant" .)