The exploding article beginning to expel a gas (6)
I believe the answer is:
ethane
'a gas' is the definition.
(ethane is a kind of gas)
'the exploding article beginning to expel' is the wordplay.
'exploding' indicates an anagram.
'article' becomes 'an' (indefinite article).
'beginning to' suggests taking the first letters.
The first letter of 'expel' is 'e'.
'the' anagrammed gives 'eth'.
'eth'+'an'+'e'='ETHANE'
(Other definitions for ethane that I've seen before include "Fuel" , "Flammable gas; eat hen (anagram)" , "Chemical" , "Ate hen (anag) - hydrocarbon gas" , "Compound" .)