"Couldn't agree more!" (4,2,4)
'couldn't agree more' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for amen that I've seen before include "Approve" , "Response to prayer" , "I agree!" , "Prayer ending" , "prayer's ending" .)
(Another definition for same here that I've seen is " Informally ditto, that's what I think too".)