A face-saving device, but the knight saw through it (5)
I believe the answer is:
visor
'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" .)