Insider's vocabulary (5)
I believe the answer is:
argot
'insider's vocabulary' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for argot that I've seen before include "Talk of thieves" , "Patois for groat" , "Jargon - patois" , "Criminal jargon" , "Special language of a groat?" .)