Gas, Cl (8)
I believe the answer is:
chlorine
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">gas</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that chlorine is a type of gas)</span><br/ ><br/ >'<span class="explanation-format__original">cl</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ ></span>
(Other definitions for chlorine that I've seen before include "Pale green gas" , "It's used to purify water" , "Yellowish-green gas" , "Chemical used as poison gas and water supply disinfectant" , "It's used to purify a swimming pool" .)