Platinum ten thieves melted down (10)

I believe the answer is:

seventieth

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'melted down' is the definition.
Both the definition and answer are adjectives. Maybe there's a link between them I don't understand?

'platinum ten thieves' is the wordplay.
'ten thieves' can be anagrammed to 'SEVENTIETH'.
But, I am unsure how the anagram is indicated.

Can you help me to learn more?

(Other definitions for seventieth that I've seen before include "platinum to celebrate" , "small fraction of whole" , "Ordinal number" , "person's birthday" .)

I've seen this clue in The Sunday Times.

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