Q: can i backup all music to iCloud so i have space on iPhone

No, backing up will not in and of itself free up storage on the device. If you are referring to itunes purchased music, you can delete and re-download those at any time.

And if you are referring to your iTunes purchases and songs from other sources that you synced with iTunes before, then you might also want to have a look at iTunes Match: https://www.apple.com/itunes/itunes-match/