Sharpness (6)
I believe the answer is:
acuity
'sharpness' is the definition.
(synonyms)
This is all the clue.
(Other definitions for acuity that I've seen before include "Showing ingenuity" , "Quick and penetrating intelligence" , "Sharp-sightedness, keenness" , "Sharpness (of e.g. vision)" , "Keenness of vision" .)