Lightning seen on bed? (5)

I believe the answer is:

sheet

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'lightning' is the definition.
(sheet lightning is a kind of lightning)

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

Can you help me to learn more?

(Other definitions for sheet that I've seen before include "Piece of paper or bedlinen" , "Mattress cover" , "Piece of linen; ship's rope" , "It usually gets blanket coverage" , "Page of postage stamps" .)

I've seen this clue in The Mirror.

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