Type of gas (7)
(Other definitions for neon that I've seen before include "Colourless gas, used in lighting" , "Gas - light" , "Gaseous element, in advertising sign say" , "illuminating agent" , "Small plant; colour" .)
'type of gas' is the definition.
(I know that methane is a type of gas)
This is all the clue.
(Other definitions for methane that I've seen before include "Main constituent of natural gas" , "Main component of natural gas" , "Get fuel" , "simplest hydrocarbon" , "Marsh-gas" .)