A regional dialect of a language (6)
I believe the answer is:
patois
'a regional dialect of a language' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for patois that I've seen before include "Kind of dialect, from French" , "Regional (French) dialect" , "Local dialect heard in patios" , "Dialect or jargon" , "A regional dialect of a language" .)