Crossword Genius

An imperative in confidentiality agreement? Who knows? (3,5,3,4)

Ross

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," .)

I've seen this clue in The Times.
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