Bag you find in the tool box (4)

I believe the answer is:

nail

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'bag' is the definition.
(thesaurus)

I can't explain the rest of the clue.

(Other definitions for nail that I've seen before include "Catch - fastener" , "Fastening device" , "Horny part of the digit" , "Fasten - detect and expose" , "Measure of cloth" .)

I've seen this clue in the Evening Standard.

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