Agreement on wonderful piano (7,5)

I believe the answer is:

concert grand

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'agreement on wonderful piano' is the definition.
'concert grand' can be an answer for 'piano' (concert grand is a kind of piano). I am not certain of the 'agreement on wonderful' bit.

'on wonderful piano' is the wordplay.
'on' is a charade indicator (letters next to each other).
'wonderful' becomes 'grand' (I've seen this in another clue).
'piano' becomes 'concert piano' (concert piano is a kind of piano).
'grand' put after 'concert' is 'CONCERT GRAND'.

Can you help me to learn more?

(Another definition for concert grand that I've seen is " Large piano".)

I've seen this clue in The Daily Express.

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