Thoughtless young woman weighed down by remorse on vacation (6)
I believe the answer is:
remiss
'thoughtless' is the definition.
(thesaurus)
'young woman weighed down by remorse on vacation' is the wordplay.
'young woman' becomes 'miss' (title for a young woman).
'weighed down by' says to put letters next to each other (in a down clue, some letters go on top of others).
'on vacation' suggests removing the centre.
'remorse' with its centre removed is 're'.
'miss' put after 're' is 'REMISS'.
(Other definitions for remiss that I've seen before include "Lacking care to duty" , "Negligent like misers" , "Negligent, lax" , "Careless concerning a girl?" , "'Careless, negligent (6)'" .)