Start off with specialist language, describing gaseous element (5)
I believe the answer is:
argon
'gaseous element' is the definition.
(I've seen this before)
'start off with specialist language describing' is the wordplay.
'start' becomes 'go' (synonyms).
'off with specialist language' becomes 'arn' (I can't justify this - if you can you should give a lot more credence to this answer).
'describing' is an insertion indicator (describe can mean to form the outline of).
'go' inserted inside 'arn' is 'ARGON'.
Can you help me to learn more?
(Other definitions for argon that I've seen before include "Noble gas used in light bulbs" , "An insert gas to groan about" , "Noble gas, Ar" , "Gas used in electric lights" , "rare gas" .)