Doom to changing demeanour (4)
I believe the answer is:
mood
'demeanour' is the definition.
'doom to changing' is the wordplay.
'to changing' is an anagram indicator.
'doom' with letters rearranged gives 'MOOD'.
(Other definitions for mood that I've seen before include "Feeling of doom" , "Emotional state" , "Disposition" , "Frame of mind" , "Low spirits" .)