Blank space for you, dear solver! (6,3)

I believe the answer is:

letter box

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

But I can't explain it at all! Could you help me understand?

(Other definitions for letter box that I've seen before include "For deposit of mail" , "Slot in a front door" , "Opening to receive mail" .)

I've seen this clue in The Hindu.

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