Reveal youngster's opening the box (5)

I believe the answer is:

telly

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'the box' is the definition.
(I've seen this before)

'reveal youngster's opening' is the wordplay.
'reveal' becomes 'tell' (I've seen this before).
'youngster's opening' becomes 'y' (I can't justify this - if you can you should believe this answer much more).
'tell'+'y'='TELLY'

Can you help me to learn more?

(Other definitions for telly that I've seen before include "One of the media" , "Familiar name for TV" , "- - Savalas" , "TV (colloq.)" , "'The box' (colloquial)" .)

I've seen this clue in The Telegraph.

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