"May it be so” (4)
I believe the answer is:
amen
'may it be so' 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 "Final word in prayer" , "prayer's ending" , "Prayerful last word" , "Word of approval" , "Word at the end of a prayer" .)