Crossword Genius

It's not private correspondence so slit the envelope (4,6)

Ross

I believe the answer is:

open letter

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'slit the envelope' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

'it's not private correspondence' is the wordplay.
'it's not private' becomes 'open' (I can't explain this - if you can you should give a lot more credence to this answer).
'correspondence' becomes 'letter' (I've seen this before).
'open'+'letter'='OPEN-LETTER'

'so' is the link.

Can you help me to learn more?

(Other definitions for open letter that I've seen before include "Written missive intended for general consumption" , "public protest" , "Not a secret missive" , "widely-publicised protest" , "No confidential correspondence" .)

I've seen this clue in The Globe and Mail.
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