Female extraterrestrial flat out on the bed (5)

I believe the answer is:

sheet

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'flat out on the bed' is the definition.
I can't tell whether this defines the answer.

'female extraterrestrial' is the wordplay.
'female' becomes 'she' (I've seen this in another clue).
'extraterrestrial' becomes 'et' (I've seen this in other clues).
'she'+'et'='SHEET'

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(Other definitions for sheet that I've seen before include "Bed cover" , "Item of bedlinen" , "Set of unseparated stamps" , "Piece of writing paper" , "Whence to sing hymns?" .)

I've seen this clue in the Evening Standard.

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