Dross incorporating new vernacular (5)
I believe the answer is:
slang
'vernacular' is the definition.
(I've seen this before)
'dross incorporating new' is the wordplay.
'dross' becomes 'slag' (synonyms).
'incorporating' is an insertion indicator.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'slag' enclosing 'n' is 'SLANG'.
(Other definitions for slang that I've seen before include "Vernacular" , "Vulgar language" , "colloquialisms" , "Patois" , "Non-standard vocabulary" .)