Popular unfounded belief in childhood (7)
I believe the answer is:
infancy
'childhood' is the definition.
(thesaurus)
'popular unfounded belief' is the wordplay.
'popular' becomes 'in'.
'unfounded belief' becomes 'fancy' (I've seen this before**).
'in'+'fancy'='INFANCY'
'in' is the link.
(Other definitions for infancy that I've seen before include "beginning" , "Early years" , "Early stages of" , "First stage of life" , "minority" .)