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Footballer should be so alert (2,3,4)

Ross

I believe the answer is:

on the ball

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'alert' is the definition.
(on the ball can mean alert or paying attention)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for on the ball that I've seen before include "Alert - informed" , "players sometimes sign name here" , "Ready to take initiative" , "Informally alert or competent, like good soccer player" , "dribbling perhaps?" .)

I've seen this clue in the Sydney Morning Herald.
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