Optical shutter? (6)

I believe the answer is:

eyelid

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

I don't understand the remainder of the clue.

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(Other definitions for eyelid that I've seen before include "Ocular shutter" , "Ocular covering" , "Cover for organ of sight" , "It blinks" , "Skin over viewer" .)

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

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