No task for a failure (8)
I believe the answer is:
omission
'a failure' is the definition.
(omission is a kind of failure)
'no task' is the wordplay.
'no' becomes 'o' (resembles 0 - 'no' can mean 'zero').
'task' becomes 'mission' (I've seen this before).
'o'+'mission'='OMISSION'
'for' is the link.
(Other definitions for omission that I've seen before include "Failure to include" , "Something left out or not done" , "Exclusion" , "lapse" , "It may create space" .)