Unhappiness due to a slump (10)
I believe the answer is:
depression
I believe this clue is a double definition.
'unhappiness' is the first definition.
(depression is a kind of unhappiness)
'a slump' is the second definition.
(I know that slump can be written as depression)
'due to' is the link.
(Other definitions for depression that I've seen before include "valley" , "despair" , "Pit" , "gloom" , "mental anguish" .)