Promise to give one a hand (7)
I believe the answer is:
betroth
'promise' is the definition.
(I know that betroth is a more specific form of the action vow)
'one a hand' is the wordplay.
I cannot really understand how this works, but
'one' could be 'o' (I've seen this in other clues) and 'o' is found in the answer.
'hand' could be 'r' (abbreviation for right, as in the right-hand side) and 'r' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'to give' acts as a link.
Can you help me to learn more?
(Other definitions for betroth that I've seen before include "Engage to be married" , "Promise to marry" , "contract" , "Promise to match" , "Affiance (one person to another)" .)