Attach a main element (6)

I believe the answer is:

staple

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

I don't understand the rest of the clue.

(Other definitions for staple that I've seen before include "essential" , "means of keeping papers together" , "Fundamental" , "Important; paper fastener" , "Short U-shaped paper fastener" .)

I've seen this clue in The Telegraph.

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