Youth in case once led astray (11)
I believe the answer is:
adolescence
'youth' is the definition.
(I've seen this before)
'case once led astray' is the wordplay.
'astray' indicates anagramming the letters.
'case'+'once'+'led'='caseonceled'
'caseonceled' is an anagram of 'ADOLESCENCE'.
'in' is the link.
(Other definitions for adolescence that I've seen before include "Transition to adulthood" , "Period of strong growth" , "Puberty" , "Period before adulthood" , "Period of youth" .)