noun. a legal doctrine that portions of copyrighted materials may be used without permission of the copyright owner provided the use is fair and reasonable, does not substantially impair the value of the materials, and does not curtail the profits reasonably expected by the owner.Based on that definition, we think you'll be fine using a single excerpt without permission (you can get more fair use here). We add our usual disclaimer: fair use is a defense with the final outcome determined by a judge.
If you're uncomfortable relying on fair use ... You can review Merriam Webster's copyright policy and seek permission as provided. If you don't want to ask for permission or rely on fair use, perhaps your definition exists in one of these public domain dictionaries (scroll down).
The real world ... A lawsuit over a single definition would not be a brilliant public relations strategy. Dictionary publishers concentrate their legal arsenal on those who purloin groups of definitions.