Slump causing gloom (10)
I believe the answer is:
depression
This clue is a double definition.
'slump' is the first definition.
('depression' can be a synonym of 'slump')
'gloom' is the second definition.
(I've seen this before)
'causing' is the link.
(Other definitions for depression that I've seen before include "Pit" , "Nineteen twenty nine financial slump" , "Blues" , "valley" , "Bad times" .)