Collar that has to be filed? (4)

I believe the answer is:

nail

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'collar' is the definition.
(I know that collar can be written as nail)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for nail that I've seen before include "Fastening device" , "Metal fastener" , "for the carpenter?" , "one under the hammer" , "metal fixture" .)

I've seen this clue in The Guardian.

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