On the ball (5)
(Other definitions for alert that I've seen before include "In state of watchfulness" , "Vigilant - warning" , "Warning signal" , "sharp-witted" , "Put on guard, warn" .)
(Other definitions for alive that I've seen before include "Quick - breathing" , "Still kicking, say" , "Animated and kicking" , "answer, as it happens" , "Go or walk briskly" .)