Promise victory to old wife (3)
I believe the answer is:
vow
'promise' is the definition.
(I know that vow is a type of oath)
'victory to old wife' is the wordplay.
'victory' becomes 'v'.
'to old' becomes 'o' (common abbreviation eg in OE for Old English).
'wife' becomes 'w' (abbreviation).
'v'+'o'+'w'='VOW'
(Other definitions for vow that I've seen before include "Promise, swear" , "Solemn pledge" , "Oath or solemn promise" , "Pledge, take oath" , "Plight" .)