Strong alloy (4,4)
(Other definitions for cast iron that I've seen before include "safe" , "Unarguable; alloy" , "Brittle alloy" , "Certain" , "difficult to defeat" .)
'strong 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 "Strength, determination" , "nerve" , "May be cold" , "Structural metal alloy" , "Make hard and unfeeling" .)