Encountered article probing helium and a greenhouse gas (7)
I believe the answer is:
methane
'a greenhouse gas' is the definition.
(I've seen this in another clue)
'encountered article probing helium' is the wordplay.
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'article' becomes 'an' (indefinite article).
'probing' indicates putting letters inside (probe can mean to poke or prod into).
'helium' becomes 'he' (He is the chemical symbol for helium).
'an' inserted into 'he' is 'hane'.
'met'+'hane'='METHANE'
'and' acts as a link.
(Other definitions for methane that I've seen before include "Marsh-gas" , "compound" , "Naturally occurring flammable gas" , "simplest hydrocarbon" , "Greenhouse gas produced by cows" .)