Something one might file away secure? (4)
I believe the answer is:
nail
This clue is a double definition.
'something one might file away' is the first definition.
(as in filing one's nails)
'secure?' is the second definition.
(I've seen this before)
(Other definitions for nail that I've seen before include "Hammered pin" , "Small metal spike" , "Hammer's target" , "Metal spike with head" , "Bring down" .)