Essential — lively (5)
I believe the answer is:
vital
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">essential</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ >'<span class="explanation-format__original">lively</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that lively can be written as vital)</span><br/ ><br/ ></span>
(Other definitions for vital that I've seen before include "Extremely important, essential to success" , "Desperately important" , "Of the greatest importance" , "Critical; life-sustaining" , "Spirited" .)