Crossword Genius

Abandon castle when king's left protected by another piece (6)

Ross

I believe the answer is:

maroon

'abandon' is the definition.
(marooning is a kind of abandoning)

I don't understand the remainder of the clue.

(Other definitions for maroon that I've seen before include "Isolate uncomfortably" , "Source of report" , "Slaves' descendant" , "Very light - dark red - strand" , "one equipped to make a report" .)

I've seen this clue in The Guardian.
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