Footballer should be so alert (2,3,4)
I believe the answer is:
on the ball
'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?" .)