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A face-saving device, but the knight saw through it (5)

Ross

I believe the answer is:

visor

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'a face-saving device' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for visor that I've seen before include "Front piece of a helmet" , "One protecting" , "Movable front part of helmet" , "Part of suit of armour" , "Face-protector" .)

I've seen this clue in the Evening Standard.
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