It's not private correspondence so slit the envelope (4,6)
I believe the answer is:
open letter
'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" .)