A very old wife making one swear (4)
I believe the answer is:
avow
'one swear' is the definition.
(I've seen this before)
'a very old wife' is the wordplay.
'a very' becomes 'av' (I've seen this in other clues).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'wife' becomes 'w'.
'av'+'o'+'w'='AVOW'
'making' is the link.
(Other definitions for avow that I've seen before include "Declare - affirm" , "Solemnly declare" , "Affirm, swear" , "Strongly maintain" , "Affirm, admit" .)