Change medium, it getting hard for metalworker

I believe the answer is:

coppersmith

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'metalworker' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps there's a link between them I don't understand?

'change medium it getting hard' is the wordplay.
'change' becomes 'coppers' (I can't justify this - if you can you should believe this answer much more).
'medium' becomes 'm' (as in UK clothing sizes).
'getting' is a charade indicator (letters next to each other).
'hard' becomes 'h'.
'coppers'+'m'+'it'+'h'='COPPERSMITH'

'for' is the link.

Can you help me to learn more?

(Another definition for coppersmith that I've seen is " Craftsman".)

I've seen this clue in The Telegraph.

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