What was that you said hospitably to the departing guest? (4,5)

I believe the answer is:

come again

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

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

(Other definitions for come again that I've seen before include "Please clarify" , "I don't understand that" , "eh?" , "You're welcome to return - pardon?" , "I'm sorry?" .)

I've seen this clue in the Evening Standard.

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