Young ladies returning, having adopted Northern jargon (5)
I believe the answer is:
slang
'jargon' is the definition.
(I know that jargon can be written as slang)
'young ladies returning having adopted northern' is the wordplay.
'young ladies' becomes 'gals' ('gal' can be a synonym of 'young lady').
'returning' is a reversal indicator.
'having adopted' means one lot of letters goes inside another.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'gals' reversed gives 'slag'.
'slag' enclosing 'n' is 'SLANG'.
(Other definitions for slang that I've seen before include "Vulgar language" , "colloquialisms" , "Vernacular" , "manner of speaking" , "Vulgar parlance" .)