Swear like a nun! (3)

I believe the answer is:

vow

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'swear' is the definition.
(thesaurus)

I don't understand the rest of the clue.

(Other definitions for vow that I've seen before include "Solemn pledge" , "Oath or solemn promise" , "Binding promise" , "Promise, swear" , "Pledge, take oath" .)

I've seen this clue in the Evening Standard.

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