Working prior to article in which dancers appear 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 don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for on the ball that I've seen before include "players sometimes sign name here" , "Wide awake" , "umpire may look for suspicious scratches here" , "Aware, alert" , "having just received a pass?" .)