Final word (5)
'final word' is the definition.
(last word in a prayer)
This is the entire clue.
(Other definitions for amen that I've seen before include "Word of approval" , "Final word in prayer" , "Religiously 11 [THE LASTWORD]" , "I agree!" , "Word of agreement" .)
(Another definition for sayso that I've seen is " rumour".)