Crossword Genius

Door opening, Spooner discussing enhanced security system? (6,3)

Ross

I believe the answer is:

letter box

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

'door opening' is the definition.

'spooner discussing enhanced security system?' is the wordplay.
I cannot really understand how this works, but
'security' could be 'letter' (letter security is a kind of security) and 'letter' is found in the answer.
The remaining letters 'box' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

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

I've seen this clue in The Guardian.
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