Between ourselves, what's said here only applies to the publishing business in the classified section (3,3,6)
I believe the answer is:
off the record
'between' is the definition.
Both the answer and definition are adverbs. Maybe you can see a link between them that I don't see?
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for off the record that I've seen before include "Confidential - not for publication" , "unprintable" , "Where the rejected tracks are kept" , "not publicly aired" , "not intended for release?" .)