Youngster notes letter (6)
I believe the answer is:
lambda
'letter' is the definition.
(letter of the Greek alphabet)
'youngster notes' is the wordplay.
'youngster' becomes 'lamb' (a lamb is a young sheep).
'notes' becomes 'da' ().
'lamb'+'da'='LAMBDA'
(Other definitions for lambda that I've seen before include "character from Greece" , "part of the skull" , "Teasing entertainer" , "Foreign character" , "The 11th letter of the Greek alphabet" .)