Ability to judge quickly and well (6)
I believe the answer is:
acumen
'ability to judge quickly and well' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for acumen that I've seen before include "Shrewdness, good for business" , "Astuteness, penetration" , "Ability to judge quickly and well" , "Cleverness in business" , "The ability to make good and quick decisions" .)