Fastener is essential (6)
I believe the answer is:
staple
'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" .)