Some hours which minutes on piano might stretch into? (9)
I believe the answer is:
afternoon
'some hours' is the definition.
(hours of the day)
I don't understand the remainder of the clue.
(Other definitions for afternoon that I've seen before include "No ante for this part of the day" , "Latter part of daylight hours" , "Post-midday" , "Midday to evening" , "Post midday" .)