Crossword Genius

Alert umpire will check for signs of tampering here? (2,3,4)

Ross

I believe the answer is:

on the ball

I'm a little stuck... Click here to teach me more about this clue!

'alert' is the definition.
(I've seen this before)

'umpire will check for signs of tampering here?' is the wordplay.
I cannot really understand how this works, but
'will' could be 'll' and 'll' is present in the answer.
'of' could be 'o' and 'o' is found within the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for on the ball that I've seen before include "Wide awake" , "Properly alert" , "dribbling perhaps?" , "Alert - informed" , "players sometimes sign name here" .)

I've seen this clue in The Hindu.
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