Be careful, like Wenceslas (4,3)

I believe the answer is:

look out

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'be careful' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

(Other definitions for look out that I've seen before include "Take care" , "as Wenceslas did" , "makes one be careful!" , "A view of the future" , "Be watchful" .)

I've seen this clue in The Guardian.

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