Light gas? (4)
I believe this is a double definition.
'light' is the first definition.
(I've seen this in another clue)
'gas?' is the second definition.
(neon is a gas)
(Other definitions for neon that I've seen before include "Element 10, used in lights" , "illuminating agent" , "Sign gas" , "One of the six inert gasses" , "Small plant; colour" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">light gas</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 helium that I've seen before include "... ditto: symbol He" , "Light colourless gas used in airships" , "Light colourless gas used in balloons" , "Atomic number 2" , "Second lightest element" .)