Dross incorporating new means of expression (5)
I believe the answer is:
slang
'means of expression' is the definition.
(slang language)
'dross incorporating new' is the wordplay.
'dross' becomes 'slag' (synonyms).
'incorporating' indicates putting letters inside.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'slag' placed around 'n' is 'SLANG'.
(Other definitions for slang that I've seen before include "Insult" , "form of language" , "See 15" , "Be abusive" , "Informal language used by a set of people" .)