"Clinical depression" refers to Major Depressive Disorder, which is the name given to the common set of psychiatric dysfunctions that have persistent depression as their primary symptom. That is, "Clinical depression" refers to an entire disorder (a set of symptoms grouped together in the literature) rather than as an individual symptom. This is to distingish it from "depression" used as the name of a symptom or other disorders, like bipolar.

"Clinical" depression is not depression that is caused by chemistry rather than psychology. We have no idea whose depression is caused by what, really, and chemistry/psychology probably isn't a meaningful distinction to make in this context.

Actual psychiatrists do not distinguish much between so-called "situational" depression, vs some hypothetical "100% organic" depression. That seems to be rampant on Internet message boards for some reason. A psychiatrist will ask about your life circumstances a little bit, but if you have certain symptoms, whether you perceive them as externally caused or not, they will diagnose you with Major Depression, i.e. "clinical depression."

Clinical just means that you have been diagnosed with Depression by someone with a bunch of letters after their name, as opposed to colloquial depression, which means something like 'feeling blue'. To be diagnosed you need to meet a number of criteria like 'loss of interest, fatigue' nearly every day for a long period.

As far as I know there is no expert consensus on whether depression and Depression are different shades of the same thing or qualitatively different.

'Depression is a chemical imbalance', as a model, is about on par with 'the internet is a series of tubes'. It's not flat out wrong, but it's so oversimplified as to be pretty useless for fixing or making predictions about the system.