Guarantee with absolute certainty, ultimately (8)

I believe the answer is:

warranty

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'guarantee' is the definition.
('warranty' can be a synonym of 'guarantee')

'with absolute certainty ultimately' is the wordplay.
'with' becomes 'w' (abbreviation).
'absolute' becomes 'arrant' ('arrant' can be a synonym of 'absolute').
'certainty ultimately' becomes 'y' (I am not sure about this - if you are sure you should believe this answer much more).
'w'+'arrant'+'y'='WARRANTY'

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(Other definitions for warranty that I've seen before include "Undertaking (as to quality)" , "Certificate" , "Bond" , "Quality guarantee" , "pledge" .)

I've seen this clue in The Times.

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