Melancholy valley (10)
I believe the answer is:
depression
I believe this is a double definition.
'melancholy' is the first definition.
(thesaurus)
'valley' is the second definition.
(I've seen this before)
(Other definitions for depression that I've seen before include "Blues" , "despair" , "Bad times" , "Nineteen twenty nine financial slump" , "gloom" .)