Failure of mole’s second assignment (8)
I believe the answer is:
omission
'failure' is the definition.
(omission is a kind of failure)
'mole's second assignment' is the wordplay.
'mole's second' becomes 'o' (2nd letter of 'mole').
'assignment' becomes 'mission' (mission is a kind of assignment).
'o'+'mission'='OMISSION'
'of' is the link.
(Other definitions for omission that I've seen before include "Something left out or neglected" , "type of sin" , "Oversight, piece of work neglected" , "lapse" , "Gap" .)