Course of dejection in the valley (10)
I believe the answer is:
depression
This clue could be a double definition.
'course of dejection' is the first definition.
'depression' can be an answer for 'dejection' (thesaurus). I am not certain of the 'course of' bit.
'valley' is the second definition.
(I've seen this before)
'in the' acts as a link.
Can you help me to learn more?
(Other definitions for depression that I've seen before include "Nineteen twenty nine financial slump" , "Blues" , "valley" , "Pit" , "gloom" .)