Moderate irritability (6)
I believe the answer is:
temper
This clue is a double definition.
'moderate' is the first definition.
('temper' can be a synonym of 'moderate')
'irritability' is the second definition.
('temper' can be a synonym of 'irritability')
(Other definitions for temper that I've seen before include "State of mind as regards anger (6)" , "Harden with heat and water" , "Toughen with anger" , "Modify" , "Uncontrolled anger" .)