A miserable little room will be depressing (6)
I believe the answer is:
sadden
'be depressing' is the definition.
I can't judge whether this definition defines the answer.
'a miserable little room' is the wordplay.
'a miserable' becomes 'sad' (I've seen this before).
'little room' becomes 'den' (den is a kind of room. I am not sure about the 'little' bit.).
'sad'+'den'='SADDEN'
'will' is the link.
Can you help me to learn more?
(Other definitions for sadden that I've seen before include "Disappoint" , "Make mournful" , "Cause to feel unhappy" , "Dismay" , "Make dejected" .)