An imperative in confidentiality agreement? Who knows? (3,5,3,4)
I believe the answer is:
you never can tell
'who knows?' is the definition.
I cannot understand how the rest of the clue works.
(Other definitions for you never can tell that I've seen before include ""Don't go shopping"" , "what you might see in the theatre" , "It's impossible to say" , "Play" , "Doubt expressed," .)