Fashionable whim of early childhood (7)
I believe the answer is:
infancy
'early childhood' is the definition.
('infancy' can be a synonym of 'early childhood')
'fashionable whim' is the wordplay.
'fashionable' becomes 'in' ('in' can be similar in meaning to 'fashionable').
'whim' becomes 'fancy' (I've seen this before).
'in'+'fancy'='INFANCY'
'of' is the link.
(Other definitions for infancy that I've seen before include "Early stages of" , "Babyhood" , "First stage of life" , "beginning" , "Early years" .)