Promises certain to be kept by a fool (7)
I believe the answer is:
assures
'promises' is the definition.
('assure' can be a synonym of 'promise')
'certain to be kept by a fool' is the wordplay.
'certain' becomes 'sure' (synonyms).
'to be kept by' is an insertion indicator.
'a fool' becomes 'ass' (both can mean a stupid person).
'sure' going inside 'ass' is 'ASSURES'.
(Other definitions for assures that I've seen before include "guarantees" , "Gives confidence or covers with a policy" , "Informs with certainty and confidence" , "States positively and confidently" , "Tells to dispel any doubts" .)