Hydrocarbon (7)
I believe the answer is:
methane
'hydrocarbon' is the definition.
(I know that methane is a alkane)
This is all the clue.
(Other definitions for methane that I've seen before include "Main component of natural gas" , "Greenhouse gas produced by cows" , "Firedamp" , "Combustible atmosphere" , "simplest hydrocarbon" .)