"I couldn't agree more!" (4)
I believe the answer is:
amen
'i couldn't agree more' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for amen that I've seen before include "prayer's ending" , "Name for prayer's end" , "Put name to end of payer" , "Approve" , "Word at the end of a prayer" .)