Ritual cleansing so lacking in forgiveness (8)
I believe the answer is:
ablution
'ritual cleansing' is the definition.
'so lacking in forgiveness' is the wordplay.
'lacking in' suggests deleting specific letters.
'forgiveness' becomes 'absolution' (I've seen this before).
'absolution' with 'so' taken away is 'ABLUTION'.
(Other definitions for ablution that I've seen before include "About nil (anag.)" , "Ritual washing in church" , "Toilet facility" , "Ceremonial washing of hands" , "Washing process" .)