Crossword Genius

Rich duke assuming king's role in castle (4)

Ross

I believe the answer is:

deep

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'rich' is the definition.
(I know that rich can be written as deep)

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for deep that I've seen before include "late" , "greatly involved?" , "Mysterious" , "Cavernous" , "Intense -- extreme" .)

I've seen this clue in The Independent.
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