Clear end of day, without question, revealing quickness of thought (6)

I believe the answer is:

acuity

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'quickness of thought' is the definition.
'acuity' can be an answer for 'quickness' (thesaurus). I am unsure of the 'of thought' bit.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for acuity that I've seen before include "Sharpness (e.g. of mind)" , "Sharp-sightedness, keenness" , "Quick and penetrating intelligence" , "Keenness of perception" , "Sharpness of vision or a quick intelligence" .)

I've seen this clue in The Independent.

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