"Let me know if you'll be there" initials (4)

I believe the answer is:

rsvp

(Other definitions for rsvp that I've seen before include "Please reply to the French, in short" , "Please answer the invitation, monsieur" , "Brief notation to say, answer the invitation" , "Letters inviting a reply" , "Invitation abbreviation" .)

I've seen this clue in the Universal.

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