In conflict, Macbeth loses heart and enthusiasm (6)
I believe the answer is:
warmth
'enthusiasm' is the definition.
(thesaurus)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for warmth that I've seen before include "Possibly fire" , "Intensity of emotion" , "tenderness" , "Cordiality" , "Pleasant heat" .)