Testy brilliance ousting gloomy state after setback (9)
I believe the answer is:
splenetic
'testy' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for splenetic that I've seen before include "Pencil set (anag.)" , "Often cross" , "Irritable" , "Bad-tempered, spiteful" , "Peevish" .)