Something neglected in old assignment (8)
I believe the answer is:
omission
'something neglected' is the definition.
I can't judge whether this definition defines the answer.
'old assignment' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'assignment' becomes 'mission' (mission is a kind of assignment).
'o'+'mission'='OMISSION'
'in' is the link.
Can you help me to learn more?
(Other definitions for omission that I've seen before include "cut?" , "task not done" , "A thing left out" , "Something lacking" , "Act of leaving out" .)