A gloomy little room can only be depressing (6)

I believe the answer is:

sadden

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'only be depressing' is the definition.
I can't judge whether this definition defines the answer.

'a gloomy little room' is the wordplay.
'a gloomy' becomes 'sad' (I've seen this in another clue).
'little room' becomes 'den' (den is a kind of room. I am not sure about the 'little' bit.).
'sad'+'den'='SADDEN'

'can' is the link.

Can you help me to learn more?

(Other definitions for sadden that I've seen before include "Upset" , "Dismay" , "Make dejected" , "Make melancholy, depress" , "Make unhappy or sorrowful" .)

I've seen this clue in the Evening Standard.

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