Harry, Bill and Peter became sharp (7)

I believe the answer is:

tapered

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But I can't explain it at all! Could you help me understand?

(Other definitions for tapered that I've seen before include "Narrowing to a point" , "Narrowed by red tape" , "Pointed" , "Reducing to a final point" , "Narrowing towards one end" .)

I've seen this clue in The Hindu.

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