Quickness of perception (6)
I believe the answer is:
acumen
'quickness of perception' is the definition.
(I've seen this before)
This is all the clue.
(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" .)