Crossword Genius

One that's passed over in the sorting office? (4,6)

Ross

I believe the answer is:

dead letter

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in the Evening Standard.
Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.