Faithful to sides exchanged. It’s majestic! (5)
I believe the answer is:
royal
'majestic' is the definition.
(synonyms)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for royal that I've seen before include "With kingly status" , "Kingly or queenly" , "Relating to kings/queens" , "Queen, say?" , "August" .)