Great enthusiasm or keenness (4)
I believe the answer is:
zeal
I believe this clue is a double definition.
'great enthusiasm' is the first definition.
(I've seen this in another clue)
'keenness' is the second definition.
(zeal is a kind of keenness)
(Other definitions for zeal that I've seen before include "Zest, ardour" , "Eagerness and devotion" , "Laze about with energy" , "Zing" , "Great eagerness" .)