Guaranteed to be convinced in the end (7)
I believe the answer is:
ensured
'guaranteed' is the definition.
('ensure' can be a synonym of 'guarantee')
'convinced in the end' is the wordplay.
'convinced' becomes 'sure' (similar in meaning).
'in' is an insertion indicator.
'sure' inserted into 'end' is 'ENSURED'.
'to be' acts as a link.
(Other definitions for ensured that I've seen before include "Make certain of" , "guaranteed" , "Made certain for end user" , "Determined" , "Made certain, clinched" .)