Something that's clinical is based on or connected to the study of patients. Clinical medications have actually been used by real people, not just studied theoretically.
When you hear about clinical drug trials, you'll know there are patients taking them and being observed — this type of test can be called clinical research. Another way to use this adjective is to mean "emotionally cold" or "impersonal." If you have a choice between a detached, clinical French teacher and a warm, charming one, you might be more likely to choose the latter. This second meaning of clinical, from the mid-1920s, originally meant "as unemotional as a medical report."