Idea lost in promises (10)
I believe the answer is:
impression
'idea' is the definition.
(impression is a kind of idea)
'lost in promises' is the wordplay.
'lost' is an anagram indicator.
'in'+'promises'='inpromises'
'inpromises' is an anagram of 'IMPRESSION'.
(Other definitions for impression that I've seen before include "Feeling; print" , "opinion" , "Mark" , "Imitation (of a person)" , "take-off" .)