Girls from south adopting new jargon (5)
I believe the answer is:
slang
'jargon' is the definition.
(synonyms)
'girls from south adopting new' is the wordplay.
'girls' becomes 'gals' ('gal' can be a synonym of 'girl').
'from south' shows that the letters should be reversed in order.
'adopting' is an insertion indicator (inserted letters are taken on or adopted).
'new' becomes 'n' (abbreviation as in New Testament NT).
'gals' back-to-front is 'slag'.
'slag' going around 'n' is 'SLANG'.
(Other definitions for slang that I've seen before include "manner of speaking" , "form of language" , "Patois" , "Colloquial language" , "Informal speech" .)