Slow movement moving to Toscanini when piano comes in (12)

'slow movement moving' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

'toscanini when piano comes in' is the wordplay.
I cannot quite see how this works, but
'when' could be 'on' (I've seen this in other clues) and 'on' is present in the answer.
'piano' could be 'p' (musical abbreviation) and 'p' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

'to' is the link.

Can you help me to learn more?