Sharp, shrewd (6)
I believe the answer is:
astute
I believe this clue is a double definition.
'sharp' is the first definition.
(synonyms)
'shrewd' is the second definition.
(I know that shrewd can be written as astute)
(Other definitions for astute that I've seen before include "Sharp-witted" , "Peerceptive" , "intelligent" , "'Perceptive, shrewd (6)'" , "Subtle" .)