Enthusiastic or sharp (4)
I believe the answer is:
keen
I believe this clue is a double definition.
'enthusiastic' is the first definition.
(I've seen this before)
'sharp' is the second definition.
(I know that sharp can be written as keen)
(Other definitions for keen that I've seen before include "Avid, enthusiastic" , "express sorrow" , "(Of a blade) sharp" , "Fervent" , "devoted" .)