Expression of assent, the last word (4)
I believe the answer is:
amen
I believe this clue is a double definition.
'expression of assent' is the first definition.
(I've seen this in another clue)
'the last word' is the second definition.
(last word in a prayer)
(Other definitions for amen that I've seen before include "Extremist" , "Name for prayer's end" , "Religiously 11 [THE LASTWORD]" , "Word at the end of a prayer" , "Conclusion of prayer" .)