On the ball; warning (5)
I believe the answer is:
alert
I believe this is a double definition.
'on the ball' is the first definition.
(on the ball can mean alert or paying attention)
'warning' is the second definition.
(I know that alert is a type of warning)
(Other definitions for alert that I've seen before include "Watchful and bright" , "Warning signal" , "'Attentive, vigilant (5)'" , "Put on guard, warn" , "On one's toes" .)