An iron container (5)
I believe the answer is:
steel
This might be a double definition.
'an iron' is the first definition.
(thesaurus)
'container' is the second definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
Can you help me to learn more?
(Other definitions for steel that I've seen before include "Strong metal" , "brace" , "The staying power" , "nerve" , "Determination, metal" .)