Fasten leading commodity (6)
I believe the answer is:
staple
This clue is a double definition.
'fasten' is the first definition.
(I know that staple is a kind of fasten)
'leading commodity' is the second definition.
(I've seen this before)
(Other definitions for staple that I've seen before include "Chief - paper fastener" , "Main, important" , "Paper pin for plates" , "Raw material" , "Basic commodity for which demand is constant" .)