Crossword Genius

Fastener of prime importance (6)

Ross

I believe the answer is:

staple

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'fastener' is the definition.
(staple is a kind of fastener)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for staple that I've seen before include "Fastener - principal" , "Paper pin - basic" , "Short U-shaped paper fastener" , "security device" , "means of keeping papers together" .)

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