One that's passed over in the sorting office? (4,6)
I believe the answer is:
dead letter
'office?' is the definition.
The definition and answer can be both states of affairs as well as being singular nouns.
Maybe there's an association between them I don't understand?
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for dead letter that I've seen before include "Unresponsive landlord" , "no longer observed" , "Itll not lead to prosecution" , "Ineffective rule" , "Law no longer enforced" .)