Showing acute intelligence after a strop? (5-5)

I believe the answer is:

razor-sharp

'showing acute intelligence' is the definition.
(razor-sharp can mean very intelligent or witty)

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

(Other definitions for razor-sharp that I've seen before include "Very finely honed" , "Extremely keen-edged" .)

I've seen this clue in The Guardian.

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