Youngster with excellent instrument (4,5)

I believe the answer is:

baby grand

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'instrument' is the definition.
(baby grand is a kind of instrument)

'youngster with excellent' is the wordplay.
'youngster' becomes 'baby'.
'with excellent' becomes 'grand' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'baby'+'grand'='BABY GRAND'

Can you help me to learn more?

(Other definitions for baby grand that I've seen before include "Small, non-upright piano" , "Type of piano" , "Infant piano?" , "Instrument" , "message of proud new parents?" .)

I've seen this clue in The Sun.

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