Performing with a high degree of precision, as in when one "runs a clinic
Their game is getting clinical these days!
To be depressed, as in clinical depression.
She is so clinical today, do you think she's okay?
Something that is a typical example (or a 'textbook
' example) of something such as a symptom of an illness (e.g. a runny nose & a high temperature) which often presented to a medical clinic is easily diagnosed. Clinical may be a synonym for 'textbook' or 'textbook case'.
addict quits playing because he determined it was ruining his life and a few weeks later begins heavily playing some other MMO
game - That's Clinical - a clear example of (in this case) an addictive personality who needs help
"He was clinical on and off the park today"