Iron alloy (5)
I believe the answer is:
steel
'iron alloy' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for steel that I've seen before include "Hard, strong, grey alloy" , "brace" , "Knife-sharpener" , "Make hard and unfeeling" , "Mixture of carbon and iron" .)