Wife taken in by brand's promise (5)
I believe the answer is:
swear
'promise' is the definition.
(as in swearing something is true)
'wife taken in by brand's' is the wordplay.
'wife' becomes 'w' (abbreviation).
'taken in by' means one lot of letters goes inside another.
'brand' becomes 'sear' (both can mean to burn).
'w' placed within 'sear' is 'SWEAR'.
(Other definitions for swear that I've seen before include "Promise on oath" , "Solemnly promise" , "Affirm" , "Vow; blaspheme" , "Use indecent or blasphemous language" .)