Put back the necessary file, for example (4)

I believe the answer is:

tool

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'file for example' is the definition.
(I know that file is a type of hand tool)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for tool that I've seen before include "Appliance" , "Utensil; implement" , "Device used for work" , "Person used by another" , "Practical implement" .)

I've seen this clue in The Guardian.

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