Restaurant or its customer (5)

I believe the answer is:

diner

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'restaurant' is the definition.
(diner is a kind of restaurant)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for diner that I've seen before include "Buffet car" , "Small US restaurant" , "US eatery" , "American cafe" , "One taking a meal" .)

I've seen this clue in The Mirror.

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