In the know about (4)

I believe the answer is:

upon

(Other definitions for upon that I've seen before include "Onto" , "Children's tales may begin ` `Once .... a time' '" , "Straddling" , "Immediately after" , ".... my word! I'm shocked" .)

I've seen this clue in the USA Today, the Penny Press, the Newsday, the Universal and the LA Times.

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