Fastener of prime importance (6)
I believe the answer is:
staple
'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" .)