Source of the metal for a gold key (3)

I believe the answer is:

ore

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'source of the metal' is the definition.
'ore' can be an answer for 'metal' (I've seen this in another clue). I'm not certain of the 'source of the' bit.

'gold key' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'key' becomes 'e' (a musical key).
'or'+'e'='ORE'

'for a' is the link.

Can you help me to learn more?

(Other definitions for ore that I've seen before include "Raw iron" , "Natural minerals" , "foreign money" , "14 [CURRENCY UNIT]" , "Solid rock containing metal" .)

I've seen this clue in the Evening Standard.

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