Negligent about failure (6)
I believe the answer is:
remiss
'negligent' is the definition.
(similar in meaning)
'about failure' is the wordplay.
'about' becomes 're' (regarding).
'failure' becomes 'miss' (miss is a kind of failure).
're'+'miss'='REMISS'
(Other definitions for remiss that I've seen before include "Negligent, derelict" , "Careless and inattentive" , "Failing in one's duty" , "Wrong" , "Negligent like misers" .)