Depression: it is a medical condition (7)

I believe the answer is:

colitis

I'm a little stuck... Click here to teach me more about this clue!

'a medical condition' is the definition.
The answer and definition can be both states of affairs as well as being singular nouns.
Maybe you can see a link between them that I don't see?

'depression it is' is the wordplay.
'depression' becomes 'col' (meteorological term).
'it is' becomes 'itis'.
'col'+'itis'='COLITIS'

Can you help me to learn more?

(Another definition for colitis that I've seen is " Intestinal inflammation".)

I've seen this clue in The Times.

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