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