They confused empty line with neon for flammable gas (8)
I believe the answer is:
ethylene
'flammable gas' is the definition.
(I've seen this before)
'they confused empty line with neon' is the wordplay.
'confused' indicates an anagram.
'empty' suggests removing the centre.
'with' is a charade indicator (letters next to each other).
'neon' becomes 'ne' (Ne is the chemical symbol for neon).
'line' with its middle removed is 'le'.
'they' anagrammed gives 'ethy'.
'ethy'+'le'+'ne'='ETHYLENE'
'for' is the link.
(Other definitions for ethylene that I've seen before include "compound" , "Flammable hydrocarbon gas" , "natural gas product" , "PVC component" , "It's found in coal gas, Tyne heel (anag.)" .)