Letter from youngster opening deposit account (6)
I believe the answer is:
lambda
'letter' is the definition.
(letter of the Greek alphabet)
'youngster opening deposit account' is the wordplay.
'youngster' becomes 'lamb' (a lamb is a young sheep).
'opening' says to take the initial letters.
The first letters of 'deposit account' is 'da'.
'lamb'+'da'='LAMBDA'
'from' acts as a link.
(Other definitions for lambda that I've seen before include "character from Greece" , "Greek letter - am bald (anag)" , "part of the skull" , "Steps taken" , "Greek character" .)