Change the name of a gas (7)
I believe the answer is:
methane
'a gas' is the definition.
(methane is a gas)
'change the name' is the wordplay.
'change' indicates anagramming the letters.
'the'+'name'='thename'
'thename' is an anagram of 'METHANE'.
'of' is the link.
(Other definitions for methane that I've seen before include "Main component of natural gas" , "Firedamp" , "simplest hydrocarbon" , "Get fuel" , "Combustible atmosphere" .)