Notable child regularly plays with it (2,3,4)
I believe the answer is:
on the ball
'with it' is the definition.
(both can mean alert)
'notable child regularly plays' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'plays' indicates anagramming the letters.
The alternate letters of 'child' are 'hl'.
'notable'+'hl'='notablehl'
'notablehl' is an anagram of 'ON THE BALL'.
(Other definitions for on the ball that I've seen before include "dribbling perhaps?" , "umpire may look for suspicious scratches here" , "Wide awake" , "Alert and practical - during soccer match?" , "Alert to new ideas" .)