Gas essential for life (6)
I believe the answer is:
oxygen
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">gas essential for life</span>' is the definition.<br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for oxygen that I've seen before include "Life-supporting gas" , "Life-giving gas" , "Gas, part of the air" , "A combustible element" , "Element forming part of water and air" .)