"I do,” for one (3)
I believe the answer is:
vow
'i do for one' 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 "Plight" , "Pledge, take oath" , "Oath or solemn promise" , "Promise, swear" , "Solemn pledge" .)