Capable 1950s youth keeping a fast (8)
I believe the answer is:
talented
'capable' is the definition.
(thesaurus)
'1950s youth keeping a fast' is the wordplay.
'1950s youth' becomes 'ted' (member of a 1950s subculture).
'keeping' is an insertion indicator.
'fast' becomes 'lent' (I've seen this before).
'a'+'lent'='alent'
'ted' enclosing 'alent' is 'TALENTED'.
(Other definitions for talented that I've seen before include "Showing a natural aptitude" , "Well-endowed with gifts" , "Having an unusual amount of ability" , "Possessing aptitude" , "Having special or great ability" .)