"Once — a time ...” (4)

I believe the answer is:

upon

(Other definitions for upon that I've seen before include "On tope of" , "Children's tales may begin ` `Once .... a time' '" , "Supported by (a surface)" , "Straddling" , "voucher, business stopped" .)

I've seen this clue in the USA Today, the Newsday, The Star Tribune and the King Feature Syndicate.

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