"Hear, hear!" (4)
I believe the answer is:
amen
'hear hear' is the definition.
('amen' might be said to indicate agreement)
This is all the clue.
(Other definitions for amen that I've seen before include "Prayer ending" , "prayer's ending" , "Religiously 11 [THE LASTWORD]" , "Word of approval" , "Last word of prayer" .)