"Let us know," initially (4)

I believe the answer is:

rsvp

(Other definitions for rsvp that I've seen before include "Brief request to acknowledge invitation" , "Brief notation to say, answer the invitation" , "Answer! (Fr.)" , "Please answer the invitation, monsieur" , "Answer the invitation briefly, please" .)

I've seen this clue in the Newsday and the L.A. Times Daily.

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