Letter I had from a youngster (3)
I believe the answer is:
kid
'a youngster' is the definition.
(I know that youngster can be written as kid)
'letter i had' is the wordplay.
'letter' becomes 'k' ().
'i had' becomes 'i'd' (I'd).
'k'+'id'='KID'
'from' is the link.
(Other definitions for kid that I've seen before include "Young animal or child" , "Sort of gloves" , "Josh" , "Jest" , "To fool; young animal" .)