Confirm, endorse (8)
I believe the answer is:
validate
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">confirm</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(validating is a kind of confirming)</span><br/ ><br/ >'<span class="explanation-format__original">endorse</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ ></span>
(Other definitions for validate that I've seen before include "Declare legally acceptable" , "stand by" , "Long poem" , "Ratify" , "Confirm correctness of" .)