Warranty (9)
'warranty' is the definition.
(I know that warranty can be written as guarantee)
This is all the clue.
(Other definitions for guarantee that I've seen before include "Formal assurance" , "Provide financial security for" , "Undertake" , "security system" , "Something ensuring a particular outcome" .)
(Other definitions for lifetime that I've seen before include "Seemingly an age?" , "Period from the cradle to the grave" , "Human span" , "Years" , "From birth to death" .)