Let it be so! (4)
I believe the answer is:
amen
'let it be so' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for amen that I've seen before include "Religiously 11 [THE LASTWORD]" , "Name for end to prayer" , "statement of affirmation" , "Word of approval" , "Put name to end of payer" .)