Important papers may need this for security (6)
I believe the answer is:
staple
I believe this clue is a double definition.
'important' is the first definition.
'papers may need this for security' is the second definition.
(papers might be stapled together)
(Other definitions for staple that I've seen before include "essential" , "important things to eat" , "Paper securer; main" , "Fundamental" , "Principle - metal fastener" .)