Vital gas (6)
I believe the answer is:
oxygen
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">vital gas</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for oxygen that I've seen before include "life support" , "Twist together" , "Gas that supports life" , "A constituent of water" , "Gaseous element supporting combustion" .)