Wire fastener, principal commodity (6)
I believe the answer is:
staple
I believe this is a double definition.
'wire fastener' is the first definition.
(I've seen this in another clue)
'principal commodity' is the second definition.
(I've seen this in another clue)
(Other definitions for staple that I've seen before include "Main - way to fasten paper" , "Fastening wire" , "Leading commodity" , "Paper pin - basic" , "means of keeping papers together" .)