Moderate one's demeanour (6)
I believe the answer is:
temper
This clue might be a double definition.
'moderate' is the first definition.
(I know that moderate can be written as temper)
'one's demeanour' is the second definition.
The definition and answer can be both human attributes as well as being singular nouns.
Maybe you can see an association between them that I can't see?
Can you help me to learn more?
(Other definitions for temper that I've seen before include "State of mind as regards anger (6)" , "Moderate - anger" , "Sudden outburst of anger" , "Rage" , "Fit of anger - don't lose it" .)