Wife wants absolute guarantee (7)
I believe the answer is:
warrant
'guarantee' is the definition.
('warrant' can be a synonym of 'guarantee')
'wife wants absolute' is the wordplay.
'wife' becomes 'w'.
'wants' says to put letters next to each other.
'absolute' becomes 'arrant' ('arrant' can be a synonym of 'absolute').
'w'+'arrant'='WARRANT'
(Other definitions for warrant that I've seen before include "Travel pass" , "Permit for arrest, search of premises, etc" , "Document giving authority" , "Writ for arrest" , "Call for" .)