Tranquil moon goddess changes sides (6)
I believe the answer is:
serene
'tranquil' is the definition.
(synonyms)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for serene that I've seen before include "Placid" , "Impassive" , "Calm, untroubled" , "Tranquil, unruffled" , "Peaceful and contented" .)