Most important one holds paper sheets together (6)

I believe the answer is:

staple

I'm a little stuck... Click here to teach me more about this clue!

'most important' is the definition.
'staple' can be an answer for 'important'. I'm not sure about the 'most' bit.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for staple that I've seen before include "Paper securer; main" , "security device" , "Paper pin for plates" , "Paper fastening" , "Basic commodity for which demand is constant" .)

I've seen this clue in The Times.

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