Unmarried woman supporting soldiers gets careless (6)
I believe the answer is:
remiss
'careless' is the definition.
(thesaurus)
'unmarried woman supporting soldiers' is the wordplay.
'unmarried woman' becomes 'miss' (I've seen this before).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'soldiers' becomes 're' (Royal Engineers).
'miss' after 're' is 'REMISS'.
'gets' is the link.
(Other definitions for remiss that I've seen before include "Lacking care to duty" , "Negligent, lax" , "Culpable" , "Failing in one's duty" , "Negligent, derelict" .)