Quick on the ___, swift to understand (6)

I believe the answer is:

uptake

(Other definitions for uptake that I've seen before include "Mental grasp" , "Ability to understand what is meant - some are slow on it" , "Ability to understand what is meant - some are quick on it" , "Acceptance; ability to understand" , "Level of acceptance" .)

I've seen this clue in The Sun.

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