Fair use means that while you're using the original work, you're adding something to it, or it simply doesn't offer the original value anymore. Let's say you'd use some copyrighted song in your video, but overlayed it with your commentary, interrupted it here and there (removing the audio), then it could be considered as fair use. But if you'd use a clip, let's say from a movie and added nothing to it (commentary, music, pictures, footage), it would most definitely NOT be fair use, even if you added something AFTER that content.
Fair dealing is quite different, as it considered to be including reviews, criticism, raising awarness, news... Simply put, if you used a copyrighted content and made it clear that it's for any of those, even without your input, you might be able to monetize it.
