Careless young lady attached to soldiers (6)
I believe the answer is:
remiss
'careless' is the definition.
(thesaurus)
'young lady attached to soldiers' is the wordplay.
'young lady' becomes 'miss' ('miss' can be a synonym of 'young lady'**).
'attached to' is a charade indicator (letters next to each other).
'soldiers' becomes 're' (Royal Engineers).
'miss' after 're' is 'REMISS'.
(Other definitions for remiss that I've seen before include "Wrong" , "Negligent like misers" , "Negligent, derelict" , "Neglectful" , "Negligent, slack" .)