Gas used in lighting (4)
I believe the answer is:
neon
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">gas used in lighting</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for neon that I've seen before include "One of the six inert gases" , "It may illuminate" , "Gaseous element, in advertising signs, say" , "Noble gas used in illuminated signs" , "Type of lighting" .)