Depends on WHY you're using that 30 second clip. Context is key. Also, while some singers & composers may set their own rules on how other people may use their songs, it's not entirely up to them to decide whether people use their material. In some specific cases you do not need permission to use copyrighted materials like music, but it's generally the polite thing to do anyway. For example, if a college professor decided to play a clip of the song or the song in its entirety as part of a larger class lecture on music and the arts, it's perfectly legal, as reusing music for educational purposes is protected by law. Alternatively, if a YouTuber, small or large, used a sample of a song as part of a review or response video to that song or music video, it is protected under fair use because they are providing their own original commentary and critique, which is considered a transformative work. You have to be really careful when using copyrighted material, but you can still get away with it if you are careful.