Light gas (4)
This clue 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" .)
'light gas' is the definition.
(I've seen this before)
This is the entire clue.
(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" .)