A communication from the spirit world? (4,6)

I believe the answer is:

dead letter

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'a communication' is the definition.
(dead letter is a kind of communication)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for dead letter that I've seen before include "Itll not lead to prosecution" , "Unresponsive landlord" , "no longer observed" , "Ineffective rule" , "Law no longer enforced" .)

I've seen this clue in the Evening Standard.

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