Crossword Genius

Sort of stop exchanging rook for bishop: what's left for you? (7)

Ross

I believe the answer is:

bequest

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

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for bequest that I've seen before include "One left in will" , "Something gifted in a will" , "Gift left in will" , "Endowment, legacy" , "Something left in a will (7)" .)

I've seen this clue in The Guardian.
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