Small flap in a table (3)

I believe the answer is:

tab

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'small flap' is the definition.
(I've seen this in another clue)

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

Can you help me to learn more?

(Other definitions for tab that I've seen before include "ticket" , "Tag" , "Tally of orders in bar" , "The bill in a restaurant" , "Flap; illicit drug pill (slang)" .)

I've seen this clue in the Evening Standard.

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