Mine changed in demeanour (4)
I believe the answer is:
mien
'demeanour' is the definition.
(I know that mien is a type of behaviour)
'mine changed' is the wordplay.
'changed' is an anagram indicator.
'mine' is an anagram of 'MIEN'.
'in' acts as a link.
(Other definitions for mien that I've seen before include "The carriage" , "A face like mine" , "Appearance is strangely mine" , "A face - maybe mine" , "Person's appearance or manner" .)