(Mental) sharpness (6)
I believe the answer is:
acuity
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mental sharpness</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)