Once more, a failure is lax (6)
I believe the answer is:
remiss
'lax' is the definition.
(I've seen this before)
'once more a failure' is the wordplay.
'once more' becomes 're' (I've seen this in other clues).
'a failure' becomes 'miss' (miss is a kind of failure).
're'+'miss'='REMISS'
'is' acts as a link.
(Other definitions for remiss that I've seen before include "Careless concerning a girl?" , "At fault" , "Negligent like misers" , "delinquent" , "Derelict in duty" .)