Belief in keeping new promises (10)
I believe the answer is:
impression
'belief' is the definition.
(synonyms)
'in keeping new promises' is the wordplay.
'keeping' is an insertion indicator (some letters kept inside).
'new' indicates an anagram.
'promises' is an anagram of 'mpressio'.
'in' placed around 'mpressio' is 'IMPRESSION'.
(Other definitions for impression that I've seen before include "Imitation (of a person)" , "Feeling; print" , "take-off" , "Mark" , "opinion" .)