Working with ancient Greek, endless learners must be 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)
'working with ancient greek endless learners' is the wordplay.
I cannot quite understand how this works, but
'working' could be 'on' (on can mean working or activated) and 'on' is located in the answer.
'learners' could be 'll' (the abbreviation for learner driver written twice) and 'll' is found in the answer.
This explanation may well be incorrect...
'must be' is the link.
Can you help me to learn more?
(Other definitions for on the ball that I've seen before include "Alert and focused as soccer player should be" , "Alert and practical - during soccer match?" , "Informally alert or competent, like good soccer player" , "Alert to new ideas" , "umpire may look for suspicious scratches here" .)