Fervor (4)
I believe the answer is:
zeal
'fervor' is the definition.
(I know that zeal is a type of fervor)
This is all the clue.
(Other definitions for zeal that I've seen before include "Fanaticism" , "Zest, ardour" , "Verve" , "Eagerness and devotion" , "(Excessive) fervour" .)