Certainty (9)
I believe the answer is:
assurance
'certainty' is the definition.
(I know that assurance is a type of certainty)
This is all the clue.
(Other definitions for assurance that I've seen before include "Confidence, certainty" , "(Life) cover" , "Guarantee - belief in oneself" , "what may follow life" , "Cure an ass of a feeling of certainty" .)