Guarantee, promise (6)
I believe the answer is:
assure
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">guarantee</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that guarantee is a kind of assure)</span><br/ ><br/ >'<span class="explanation-format__original">promise</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that promise can be written as assure)</span><br/ ><br/ ></span>
(Other definitions for assure that I've seen before include "Convince, promise" , "Cover with a policy" , "Guarantee, promise" , "Make (something) certain to happen" , "Convince, guarantee" .)