Careless, fail once more? (6)
I believe the answer is:
remiss
I believe this clue is a double definition.
'careless' is the first definition.
(I know that careless can be written as remiss)
'fail once more?' is the second definition.
('miss' is an answer for 'fail' so perhaps 'remiss' is OK for 'fail once more')
(Other definitions for remiss that I've seen before include "delinquent" , "Negligent like misers" , "Negligent in performance of duty" , "At fault" , "Failing in one's duty" .)