Gas initially envelops your old tip, according to reports (8)
I believe the answer is:
ethylene
'tip according to reports' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'gas initially envelops your old' is the wordplay.
'gas initially' becomes 'elene' (I can't explain this - if you can you should believe this answer much more).
'envelops' indicates putting letters inside (I've seen this in other clues).
'your old' becomes 'thy' ('thy' historically meant 'your').
'elene' enclosing 'thy' is 'ETHYLENE'.
(Other definitions for ethylene that I've seen before include "Flammable hydrocarbon gas" , "compound" , "It's found in coal gas, Tyne heel (anag.)" , "PVC component" , "natural gas product" .)