Doubly depressed - that's unfair (3,4)
I believe the answer is:
low down
'unfair' is the definition.
Both the definition and answer are adjectives. Perhaps you can see a link between them that I can't see?
'doubly depressed' is the wordplay.
'doubly' becomes 'low' (I am not sure about this - if you are sure you should believe this answer much more).
'depressed' becomes 'down' (synonyms).
'low'+'down'='LOW-DOWN'
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for low down that I've seen before include "Mean" , "Data - base" , "dope?" , "Abysmal" , "Cheap" .)