Guarantee wife complete authority, finally (8)
I believe the answer is:
warranty
'guarantee' is the definition.
(I know that guarantee can be written as warranty)
'wife complete authority finally' is the wordplay.
'wife' becomes 'w' (abbreviation).
'complete' becomes 'arrant' (old-fashioned term meaning complete or utter).
'finally' says to take the final letters.
The final letter of 'authority' is 'y'.
'w'+'arrant'+'y'='WARRANTY'
(Other definitions for warranty that I've seen before include "Undertaking (as to quality)" , "Contract" , "pledge" , "Bond" , "Guarantee given to a purchaser" .)