Mental quickness (9)

I believe the answer is:

dexterity

(Other definitions for dexterity that I've seen before include "Physical skill in handling things" , "Sleight" , "Manual skill" , "Skill in handling things, adroitness" , "Skill of manipulation" .)

I've seen this clue in The Mirror.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.