Essential, indispensable (9)
I believe the answer is:
necessary
<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">(I know that essential can be written as necessary)</span><br/ ><br/ >'<span class="explanation-format__original">indispensable</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(associated in meaning)</span><br/ ><br/ ></span>
(Other definitions for necessary that I've seen before include "Required, needed" , "Indispensable, essentional" , "Inescapable" , "Vital" , "Required to be done" .)