Keen, penetrating (8)
I believe the answer is:
incisive
I believe this is a double definition.
'keen' is the first definition.
(I know that keen can be written as incisive)
'penetrating' is the second definition.
(I know that penetrating can be written as incisive)
(Other definitions for incisive that I've seen before include "Analytical, concise" , "(Of action) quick, direct" , "Clear and cutting" , "Piercing" , "Of speech, sharp and clear" .)