Fool includes certain guarantees (7)
I believe the answer is:
assures
'guarantees' is the definition.
('assure' can be a synonym of 'guarantee')
'fool includes certain' is the wordplay.
'fool' becomes 'ass' (ass is a kind of fool).
'includes' is an insertion indicator.
'certain' becomes 'sure' (synonyms).
'ass' going around 'sure' is 'ASSURES'.
(Other definitions for assures that I've seen before include "guarantees" , "States positively and confidently" , "Informs with certainty and confidence" , "Tells to dispel any doubts" , "Gives confidence or covers with a policy" .)