Solemn pledge (3)
I believe the answer is:
vow
'solemn pledge' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for vow that I've seen before include "Promise, swear" , "Solemn pledge" , "Binding promise" , "Oath or solemn promise" , "Pledge, take oath" .)