Very keen; enthusiastic (5)
I believe the answer is:
eager
I believe this clue is a double definition.
'very keen' is the first definition.
(I've seen this before)
'enthusiastic' is the second definition.
(similar in meaning)
(Other definitions for eager that I've seen before include "Keenly expectant" , "Swiss mountain" , "Keenly interested" , "Keen, avid" , "Earnest" .)