An error of management (9)
I believe the answer is:
oversight
This clue is a double definition.
'an error' is the first definition.
(oversight is a kind of error)
'management' is the second definition.
(overseeing something)
'of' is the link.
(Other definitions for oversight that I've seen before include "Mistake resulting from inattention" , "act of negligence" , "Inadvertent mistake" , "what supervisor has" , "Neglect" .)