She's vowed to be good (3)
I believe the answer is:
nun
'good' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for nun that I've seen before include "Mother Superior, perhaps" , "Devout woman" , "One who's taken vows" , "Convent dweller" , "Woman respecting order" .)