Crossword Genius

An ivory castle perhaps (8)

Ross

I believe the answer is:

chessman

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This could be a double definition.

'an ivory' is the first definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

'castle perhaps' is the second definition.
(I know that rook is a type of chessman)

Can you help me to learn more?

(Other definitions for chessman that I've seen before include "Eg, bishop, knight" , "Eg, bishop or rook" , "Playing piece" , "queen, perhaps" , "that's played on board" .)

I've seen this clue in The Globe and Mail.
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