Loyalty to the king (5)

I believe the answer is:

troth

(Other definitions for troth that I've seen before include "Plight one's ..., make a solemn promise in marriage" , "Plight one's . . . . . - promise to marry" , "Pledge, especially of marriage" , "proposal" , "old-fashioned faith" .)

I've seen this clue in The Wall Street Journal.

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