Promise solemnly (3)

I believe the answer is:
vow
'promise solemnly' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for vow that I've seen before include "Pledge, take oath" , "Solemn pledge" , "Plight" , "Binding promise" , "Oath or solemn promise" .)