"Amen!” (4)
'amen' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for yeah that I've seen before include "The old expression of surprise" , "positive response?" , "I agree" , "Yes (informally)" , "quite casually" .)
(Other definitions for preach that I've seen before include "Publicly proclaim" , "Proclaim (a religious message)" , "Speak from the pulpit" , "Take the moral high ground" , "Moralise" .)