(Mental) sharpness (6)
I believe the answer is:
acuity
'mental sharpness' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for acuity that I've seen before include "Sharpness of vision or a quick intelligence" , "Showing ingenuity" , "Keenness of perception" , "Keenness of vision" , "Mental sharpness" .)