Language is initially just gas (6)
I believe the answer is:
jargon
'language' is the definition.
(I know that jargon is a type of language)
'initially just gas' is the wordplay.
'initially' says to take the initial letters.
'gas' becomes 'argon' (argon is a gas).
The initial letter of 'just' is 'j'.
'j'+'argon'='JARGON'
'is' is the link.
(Other definitions for jargon that I've seen before include "Words and expressions used within a particular group" , "Specialist language" , "Cant" , "variety of zircon" , "Professional terminology" .)