Youth in want (7)
I believe the answer is:
infancy
'youth' is the definition.
(I've seen this before)
'in want' is the wordplay.
'want' becomes 'fancy' (fancying is a kind of wanting).
'in'+'fancy'='INFANCY'
(Other definitions for infancy that I've seen before include "Early stage of development" , "First stage of life" , "Early stages of" , "Early years" , "State of early childhood" .)