Like coal or oil (5,4)

I believe the answer is:

black gold

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'oil' is the definition.
(I've seen this in another clue)

'like coal or' is the wordplay.
'like coal' becomes 'black' (I am not sure about this - if you are sure you should believe this answer much more).
'or' becomes 'gold' ('or' is the heraldic term for gold).
'black'+'gold'='BLACK GOLD'

Can you help me to learn more?

(Another definition for black gold that I've seen is " Oil".)

I've seen this clue in The Sunday Times and The Times.

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