Some musicians routinely purchase an extra seat to travel with their instrument. It normally works, but it isn't unusual to see a report of someone getting refused while boarding. Just be aware that you could make arrangements with the airline and do everything right but still run into a problem.
https://www.thestrad.com/news/couple-with-cello-removed-from-american-airlines-flight/16413.article
https://simpleflying.com/klm-under-fire-cello-cabin-refusal/
This one was a surprise because Sheku Kanneh-Mason is one of the biggest name classical cellists today.
https://www.classicfm.com/artists/sheku-kanneh-mason/barred-ba-flight-cello-call-airline-protocol/