Moderate bad mood (6)
I believe the answer is:
temper
I believe this clue is a double definition.
'moderate' is the first definition.
('temper' can be a synonym of 'moderate')
'bad mood' is the second definition.
(I've seen this before)
(Other definitions for temper that I've seen before include "Mood (often bad)" , "Uncontrolled anger" , "Mood, maybe angry" , "Volatile anger - don't lose it" , "Toughen with anger" .)