Jargon, unlimited jargon before chat's conclusion (5)
I believe the answer is:
argot
'jargon' is the definition.
(argot can mean jargon or slang)
'unlimited jargon before chat's conclusion' is the wordplay.
'unlimited' means to look at the middle letters (discard the edges or 'limits' of the word).
'before' means one lot of letters go next to another.
'conclusion' says to take the final letters.
The middle of 'jargon' is 'argo'.
The last letter of 'chat' is 't'.
'argo'+'t'='ARGOT'
(Other definitions for argot that I've seen before include "Patois, vernacular" , "Father or mother" , "Slang for a groat (5)" , "Jargon or slang (of a group)" , "secret language" .)