It needs a hot iron (6)
I believe the answer is:
solder
This might be a double definition.
'it' is the first definition.
The definition suggests a singular noun which matches the answer.
'a hot iron' is the second definition.
The definition and answer can be both substances as well as being singular nouns.
Maybe there's an association between them I don't understand?
'needs' acts as a link.
Can you help me to learn more?
(Other definitions for solder that I've seen before include "Metal bond" , "Join two metal surfaces together" , "Join two metals with a melted alloy" , "Melted alloy, used to join two metal surfaces" , "Soft alloy used to cement metals" .)