Shrewd; sharp-witted (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-witted' is the second definition.
(I've seen this before)
(Other definitions for astute that I've seen before include "Able to make accurate assessments" , "Clever and perceptive" , "Sharp-witted" , "Shrewd, canny" , "Canny and sagacious" .)