Careless, extremely rude young lady (6)
I believe the answer is:
remiss
'careless' is the definition.
(thesaurus)
'extremely rude young lady' is the wordplay.
'extremely' says to hollow out the word (remove centre letters).
'young lady' becomes 'miss' ('miss' can be a synonym of 'young lady'**).
'rude' with its centre taken out is 're'.
're'+'miss'='REMISS'
(Other definitions for remiss that I've seen before include "Negligent like misers" , "Negligent, derelict" , "Wrong" , "Neglectful" , "Careless and inattentive" .)