When something is created, it's subject to copyright automatically. Every song is subject to copyright unless it's in the public domain. You must ask permission, unless they've authorised use under a licence of some sort and you comply with that licence.
Nothing I can think of would protect you under fair dealing. If you go ahead without permission, if they find it and don't want you doing it, they'll get YouTube to take it down. That's the minimum of what they'd do.
I've actually seen some artists commenting on illegally-uploaded videos featuring their songs. One example is this one, where the guy who wrote the song (Danny Kavanagh) responds to someone accusing him of plagiarism: [link] I lol'd.