Epiphany came as our smallest denomination financially declined (3,5,7)
I believe the answer is:
the penny dropped
'epiphany came as our smallest denomination financially' is the definition.
I can't tell whether this defines the answer.
'declined' is the wordplay.
I cannot really understand how this works, but
'declined' could be 'dropped' (dropping is a kind of declining) and 'dropped' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for the penny dropped that I've seen is " what happened around 10 [AHA MOMENT] ?".)