Main thing to secure papers (6)

I believe the answer is:

staple

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'main' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

(Other definitions for staple that I've seen before include "means of keeping papers together" , "Basic commodity and paper fastener" , "Metal fixer" , "Raw material" , "U-shaped wire to fasten papers together" .)

I've seen this clue in The Guardian.

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