Moderate fit of rage (6)
I believe the answer is:
temper
I believe this clue is a double definition.
'moderate' is the first definition.
(I know that moderate can be written as temper)
'fit of rage' is the second definition.
(temper can mean an angry fit)
(Other definitions for temper that I've seen before include "Harden with heat and water" , "Bad humour" , "(Calm or furious) disposition" , "Moderate; ill humour" , "Mood (often bad)" .)