Confirm, guarantee (7)
I believe the answer is:
certify
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">confirm</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >'<span class="explanation-format__original">guarantee</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(certifying is a kind of guaranteeing)</span><br/ ><br/ ></span>
(Other definitions for certify that I've seen before include "Authorise officially" , "Declare; endorse" , "Formally confirm" .)