Dangerous gas (5)
I believe the answer is:
radon
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">dangerous gas</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 radon that I've seen before include "Noble gas, Rn" , "Radioactive gas, linked to granite" , "Gaseous element, Rn" , "A radioactive gas, symbol Rn" , "Natural gas considered a hazard to health" .)