Erase all content and setting.Delete some else's iCloud?

My uncle gave me a used iPhone 6.Everything is fine except for that. Its still signed into the past users iCloud. I want to know if I Erase all Content and setting. Will it erase the past users iCloud account from the phone?

No. You will have to sign out of it properly. Assuming it was a bought phone, they should try to return it or at least find out the credentials needed.

You can't disconnect from the past user's iCloud account unless they do it.