Quickness of perception (6)
I believe the answer is:
acumen
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">quickness of perception</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for acumen that I've seen before include "Ability to judge quickly and well" , "Keen intelligence" , "'Shrewdness, in business perhaps (6)'" , "Penetrating insight" , "show perception" .)