Element in water (8)
I believe the answer is:
hydrogen
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">element in water</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for hydrogen that I've seen before include "The commonest element" , "Combines chemically with oxygen to form water" , "Lightest gas" , "... ditto: symbol H" , "bomb-maker" .)