Careless omission (9)
I believe the answer is:
oversight
'careless omission' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for oversight that I've seen before include "Failure to notice - blunder" , "A failure to notice, an omission" , "Supervisor's charged with this" , "Neglect" , "act of negligence" .)