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