Beginning of one new idea (7)
I believe the answer is:
infancy
'beginning' is the definition.
(I've seen this before)
'of one new idea' is the wordplay.
'of one' becomes 'i' (I've seen this in other clues).
'new' becomes 'n'.
'idea' becomes 'fancy' (fancy is a kind of idea**).
'i'+'n'+'fancy'='INFANCY'
(Other definitions for infancy that I've seen before include "Early stage of development" , "Babyhood" , "Early stages of" , "minority" , "First stage of life" .)