Popular, extravagant youth (7)
I believe the answer is:
infancy
'youth' is the definition.
(I've seen this before)
'popular extravagant' is the wordplay.
'popular' becomes 'in' (something that's 'in' is fashionable).
'extravagant' becomes 'fancy' (fancy can mean extravagant).
'in'+'fancy'='INFANCY'
(Other definitions for infancy that I've seen before include "Early stages of" , "State of early childhood" , "First stage of life" , "youth" , "Babyhood" .)