A greeting for one you hadn't expected? (5)

I believe the answer is:

hello

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'a greeting' is the definition.
(I know that Hello is a greeting)

I cannot understand how the rest of the clue works.

(Other definitions for hello that I've seen before include "I say" , "A salutation" , "Look at that" , "Expression of greeting" , "Introductory greeting" .)

I've seen this clue in the Evening Standard.

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