Thoughtless regarding young woman (6)
I believe the answer is:
remiss
'thoughtless' is the definition.
(thesaurus)
'regarding young woman' is the wordplay.
'regarding' becomes 're' ('re' means regarding or about).
'young woman' becomes 'miss' (title for a young woman).
're'+'miss'='REMISS'
(Other definitions for remiss that I've seen before include "Negligent in performance of duty" , "Negligent, slack" , "Lacking attention to duty" , "Negligent like misers" , "Failing in what duty requires" .)