Insider's lingo (5)
I believe the answer is:
argot
'insider's lingo' 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 "Patois, vernacular" , "Jargon - patois" , "Special language of a groat?" , "Groat (anag)" , "Ancient Greek coin" .)