Sharp and enthusiastic (4)
I believe the answer is:
keen
I believe this is a double definition.
'sharp' is the first definition.
(I know that sharp can be written as keen)
'enthusiastic' is the second definition.
(I've seen this before)
'and' acts as a link.
(Other definitions for keen that I've seen before include "Innumerable" , "Having a fine edge to weep over the dead" , "Sharp - lament" , "Penetrating; ardent" , "cry" .)