Secret old coin colour (4,5)

I believe the answer is:

dark brown

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'secret' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?

'old coin colour' is the wordplay.
'old coin' becomes 'dark' (I can't explain this - if you can you should give a lot more credence to this answer).
'colour' becomes 'brown' (brown is a kind of colour).
'dark'+'brown'='DARK-BROWN'

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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