Something to do with a hot iron (6)
I believe the answer is:
solder
'iron' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Maybe they are linked in a way I don't understand?
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for solder that I've seen before include "Join two metals with a melted alloy" , "Join or fuse metal, using an alloy" , "Metal bond" , "Join two metal surfaces together" , "Soft alloy used to cement metal parts" .)