Glib one's grasping old lingo (6)
I believe the answer is:
patois
'lingo' is the definition.
('patois' can be a synonym of 'lingo')
'glib one's grasping old' is the wordplay.
'glib' becomes 'pat' ('pat' can be a synonym of 'glib').
'one's' becomes 'is' (Roman numeral).
'grasping' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'pat'+'is'='patis'
'patis' placed around 'o' is 'PATOIS'.
(Other definitions for patois that I've seen before include "Kind of dialect, from French" , "A regional dialect of a language" , "Form of language" , "Language of group" , "Spoken regional dialect" .)