Ensure (9)
I believe the answer is:
guarantee
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">ensure</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that ensure can be written as guarantee)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for guarantee that I've seen before include "Promising" , "Undertake" , "Written assurance, pledge" , "Something ensuring a particular outcome" , "security system" .)