Change the name for a gas (7)
I believe the answer is:
methane
'a gas' is the definition.
(methane is a kind of gas)
'change the name' is the wordplay.
'change' indicates an anagram.
'the'+'name'='thename'
'thename' anagrammed gives 'METHANE'.
'for' is the link.
(Other definitions for methane that I've seen before include "Main component of natural gas" , "Flammable gas, CH4" , "Marsh gas - the name (anag)" , "Combustible atmosphere" , "Naturally occurring flammable gas" .)