Kind of varnish giving the piano that black outward appearance (5)

I believe the answer is:

copal

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'kind of varnish giving the piano that' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

'black outward appearance' is the wordplay.
'black' becomes 'cop' (I can't explain this - if you can you should give a lot more credence to this answer).
'outward' says to hollow out the word (remove centre letters).
'appearance' becomes 'arrival' (to appear can mean to arrive at a particular place).
'arrival' with its centre taken out is 'al'.
'cop'+'al'='COPAL'

Can you help me to learn more?

(Other definitions for copal that I've seen before include "Resin used in varnish" , "Hard resin" , "Aromatic tree resin" .)

I've seen this clue in the Evening Standard.

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