The 51st Psalm (8)
I believe the answer is:
miserere
'the 51st psalm' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for miserere that I've seen before include "appeal for mercy" , "Psalm seeking mercy" , "Setting of Psalm fifty one" , "Mercy Seat" , "plea for mercy" .)