Metal cover protects the sewer (7)

I believe the answer is:

thimble

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'metal cover' is the definition.
'thimble' can be an answer for 'cover' (I've seen this before). I am unsure of the 'metal' bit.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for thimble that I've seen before include "Sewer's aid" , "Thumb thing" , "Protective item for the tailor" , "Digital protection" , "Cap to protect the finger while sewing" .)

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

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