Shutter closed for siesta? (6)

I believe the answer is:

eyelid

I'm a little stuck... Click here to teach me more about this clue!

'shutter' is the definition.
(I have seen 'Facial shutter' mean 'eyelid' so perhaps 'shutter' could also mean 'eyelid')

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for eyelid that I've seen before include "Blinking cover" , "Blinking membrane" , "Cover for viewer" , "Skin for blinking" , "Protection for organ of sight" .)

I've seen this clue in The Times.

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