Iron alloy used for brace (5)
I believe the answer is:
steel
I believe this clue is a double definition.
'iron alloy' is the first definition.
(I've seen this before)
'brace' is the second definition.
(steeling is a kind of bracing)
'used for' acts as a link.
(Other definitions for steel that I've seen before include "Make hard and unfeeling" , "Hard, strong, grey alloy" , "Strength, determination" , "brace" , "The staying power" .)