Crossword Genius

Fastener is essential (6)

Ross

I believe the answer is:

staple

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'essential' is the definition.
(similar in meaning)

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 "Paper pin - basic" , "important things to eat" , "security device" , "wool" , "Principle - metal fastener" .)

I've seen this clue in The Times and The Sun.
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