Has she vowed not to take orders? (3)

I believe the answer is:
nun
'take orders?' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'has she vowed not' is the wordplay.
I cannot quite see how this works, but
'not' could be 'un' (prefix meaning 'not') and 'un' is located in the answer.
A single letter 'n' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'to' is the link.
Can you help me to learn more?
(Other definitions for nun that I've seen before include "Letter" , "whose son entered Promised Land" , "Religious sister" , "She's made vows" , "religious person" .)
