Shrewdness that maybe came unstuck at first (6)

I believe the answer is:

acumen

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'shrewdness' is the definition.
(acumen is a kind of shrewdness)

I can't explain the remainder of the clue.

(Other definitions for acumen that I've seen before include "perspicacity" , "Insightfulness" , "Sharpness, penetration, quickness of perception" , "Shrewdness, intelligence in business" , "Shrewdness, sharpness of mind" .)

I've seen this clue in the Evening Standard.

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