The final word in a lament (4)
I believe the answer is:
amen
'the final word' is the definition.
(last word in a prayer)
'in a lament' is the wordplay.
'in' indicates the answer is hidden within the clue.
'AMEN' is hidden in the letters of 'lament'.
(Other definitions for amen that I've seen before include "Final word in prayer" , "as agreed!" , "statement of affirmation" , "I agree!" , "Churchgoer's assent?" .)