A tool in the cabinet? (4)

I believe the answer is:

file

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'a tool' is the definition.
(file is a kind of tool)

I don't understand the rest of the clue.

(Other definitions for file that I've seen before include "Line on board" , "Set of related records kept together" , "Smoothing tool - orderly batch of documents" , "Line of people, one behind the other" , "Folder; line" .)

I've seen this clue in the Evening Standard.

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