Prayerful piece of lamentation (4)

I believe the answer is:

amen

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'prayerful piece' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

'of lamentation' is the wordplay.
'of' indicates a hidden word.
'AMEN' is hidden in the letters of 'lamentation'.

Can you help me to learn more?

(Other definitions for amen that I've seen before include "Prayer ending" , "Name for end to prayer" , "Response to prayer" , "Word at the end of a prayer" , "Extremist" .)

I've seen this clue in the Evening Standard.

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