I would like to include some material which will need clearance. Is this something that needs doing before finding a publisher? Or is it just that one publisher talks to another, and authors have no business being involved in?
If you are using content that is protected under copyright, then you should make sure that you have permission to use it and/or have attributed the original source appropriately before trying to find a publisher. The act of trying to sell your own work which contains somebody else's copyrighted content could be construed as a violation of that copyright. It is highly unlikely that any publisher will even consider your work, let alone attempt to pursue the permission or release from the original source.
When I say "and/or" have it attributed, I mean to say that you may be able to use the copyrighted material with only an attribution to the creator of the material, and not explicit permission. This would ultimately depend on how you are using the content and how much of it you are using. If it falls within the guidelines of Fair Use, then all you need to do is give credit to the original source. (The link will provide some detailed examples.) If your usage of the copyrighted material goes beyond this, then you will need to seek permission and give credit to the original source.
Fair use is a limitation and exception to the exclusive right granted by copyright law to the author of a creative work. In United States copyright law, fair use is a doctrine that permits limited use of copyrighted material without acquiring permission from the rights holders.