Lancelot for example loses his head in the dark (5)

I believe the answer is:

night

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'the dark' is the definition.
(it's dark at nighttime)

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

(Other definitions for night that I've seen before include "See 9" , "8d [DARKNESS]" , "The dark time" , "''Twas the ... before Christmas, when all through the house / Not a creature was stirring, not even a mouse''" , "follow-up to 16 [TWILIGHT]" .)

I've seen this clue in The Globe and Mail.

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