Essential to life (5)
I believe the answer is:
vital
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">essential to life</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for vital that I've seen before include "Highly important" , "Essential, life-giving" , "Essential to life, indispensable" , "Characteristic of life" , "Critical, necessary" .)