Instrument excellent for an infant (4,5)
I believe the answer is:
baby grand
'instrument' is the definition.
(baby grand is a kind of instrument)
'excellent for an infant' is the wordplay.
'excellent' becomes 'grand' (I am not sure about this - if you are sure you should believe this answer much more).
'for' is a charade indicator (letters next to each other) (I've seen this in other clues).
'an infant' becomes 'baby' ('baby' can be a synonym of 'infant').
'grand' put after 'baby' is 'BABY GRAND'.
Can you help me to learn more?
(Other definitions for baby grand that I've seen before include "Infant piano?" , "Type of piano" , "message of proud new parents?" , "Instrument" , "Smaller piano" .)