Make amends with key immediately (2,4)
I believe the answer is:
at once
'immediately' is the definition.
(I've seen this before)
'make amends with key' is the wordplay.
'make amends' becomes 'atone' (to atone is to make amends for a wrong deed).
'with' is an insertion indicator.
'key' becomes 'c' (musical key).
'atone' placed around 'c' is 'AT ONCE'.
(Other definitions for at once that I've seen before include "'Immediately (2,4)'" , "This minute!" , "Right now, this instant" , "simultaneously" , "Immediately, this instant" .)