Basic; lively (5)
I believe the answer is:
vital
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">basic</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >'<span class="explanation-format__original">lively</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ ></span>
(Other definitions for vital that I've seen before include "Essential; lively" , "Essential to life or critically important" , "Most important, like certain body organs" , "Are such statistics" , "Of the greatest importance" .)