Context would really help, but I’ll guess. You’ve understood “chill” correctly, so we need to understand “clinically”. “Clinically” can be used as an intensifier in describing someone’s physical or emotional state — the idea is that a doctor could evaluate this person, perhaps run some medical tests, and then diagnose the person as “chill”. Since the doctor declared this person “chill” in a clinical setting (by direct observation), this person can be considered “clinically chill”. Of course, this is just meant as a funny figure of speech, as nobody actually goes to a doctor for such a diagnosis. A more common expression is “clinically depressed”, meaning a person is not just feeling blue, but that he has been medically diagnosed with depression.
Edited for a typo.