Two weeks by fort in darkness (5)

I believe the answer is:

night

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'darkness' is the definition.
(night is a period of darkness)

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

(Other definitions for night that I've seen before include "Darkness; see you in the morning" , "Dark hours" , "Time without 10" , "follow-up to 16 [TWILIGHT]" , "End of the day" .)

I've seen this clue in The Guardian.

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