Open and close inside short period (9)

I believe the answer is:

overnight

'short period' is the definition.
(only a single night)

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

(Other definitions for overnight that I've seen before include "until tomorrow" , "Until morning; suddenly" , "from one day to the next" , "darkness" , "Lightning" .)

I've seen this clue in The Guardian.

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