Shrewd, sharp (6)
I believe the answer is:
astute
This clue is a double definition.
'shrewd' is the first definition.
(I know that shrewd can be written as astute)
'sharp' is the second definition.
(I know that sharp can be written as astute)
(Other definitions for astute that I've seen before include "Wise" , "Canny and sagacious" , "Sharp-witted" , "Shrewd, canny" , "Sharp and clever" .)