Moderate one's mood (6)
I believe the answer is:
temper
This could be a double definition.
'moderate' is the first definition.
(I know that moderate can be written as temper)
'one's mood' is the second definition.
'temper' can be an answer for 'mood' (temper is one's mood or state of mind). I am not sure about the 'one's' bit.
Can you help me to learn more?
(Other definitions for temper that I've seen before include "Make hard by heating and cooling, like iron" , "Moderate; ill humour" , "Tendency to become angry easily" , "Tendency to anger" , "(Calm or furious) disposition" .)