Once, in the early afternoon perhaps, it came back to me (2,3,4)
I believe the answer is:
at one time
'once' is the definition.
(I know that once can be written as at one time)
'early afternoon perhaps it came back to me' is the wordplay.
'early' indicates taking the first letters (I've seen 'earliest of' mean this).
'perhaps' becomes 'tone' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'came back' says the letters should be written in reverse.
'to' says to put letters next to each other.
The first letter of 'afternoon' is 'a'.
'it' back-to-front is 'ti'.
'a'+'tone'+'ti'+'me'='AT ONE TIME'
'in the' acts as a link.
Can you help me to learn more?
(Other definitions for at one time that I've seen before include "together" , "in concert" , "Taking place simultaneously" , "Formerly" , "in the past" .)