"You said it!" (4)
'you said it' 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 "Religiously 11 [THE LASTWORD]" , "Word at the end of a prayer" , "Last word of prayer" , "Approve" , "Extremist" .)
(Other definitions for preach that I've seen before include "Moralise" , "Proclaim (religion)" , "Sermonise" , "Do as Amos Starkadder did" , "Proclaim (a religious message)" .)
(Other definitions for agreed that I've seen before include "'Concurred, assented (6)'" , "Consented, settled" , "Of one mind" , "In accord" , "Acquiesed, concurred" .)