Sword made from thin sheet of metal (4)
I believe the answer is:
This clue is a double definition.
'sword' is the first definition.
(foil is a kind of sword)
'thin sheet of metal' is the second definition.
(as in tin foil)
'made from' acts as a link.
(Other definitions for foil that I've seen before include "Baffle" , "Repulse" , "Parry" , "blunt weapon" , "Blunt sword with a button on the end" .)