Once accepts Timothy is not late (2,4)
I believe the answer is:
on time
'is not late' is the definition.
(I've seen this in another clue)
'once accepts timothy' is the wordplay.
'once' becomes 'one' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'accepts' indicates putting letters inside.
'timothy' becomes 'tim' (I've seen this in other clues).
'one' placed around 'tim' is 'ON TIME'.
Can you help me to learn more?
(Other definitions for on time that I've seen before include "promptly" , "according to schedule" , "(Of train) not late" , "Prompt" , "Punctual, not late" .)