Confidentially! (7,3,3,2)
I believe the answer is:
between you and me
'confidentially' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for between you and me that I've seen before include "Don't tell anybody else!" , "not going any further" , "our inheritance may be so divided" , "Confidentially" , "said in private" .)