Being in love changes demeanour (9)
I believe the answer is:
enamoured
'being in love' is the definition.
(I've seen this before)
'changes demeanour' is the wordplay.
'changes' indicates anagramming the letters.
'demeanour' with letters rearranged gives 'ENAMOURED'.
(Other definitions for enamoured that I've seen before include "In love with - named rou (anag)" , "Greatly taken with" , "Charmed" , "Bewitched" , "Our mean Ed (anag.)" .)