Strange mercy, one ritual (8)
I believe the answer is:
ceremony
'ritual' is the definition.
(I've seen this before)
'strange mercy one' is the wordplay.
'strange' indicates an anagram.
'mercy'+'one'='mercyone'
'mercyone' is an anagram of 'CEREMONY'.
(Other definitions for ceremony that I've seen before include "Formal event performed on a special occasion" , "Rite, ritual or formal act" , "Religious rite" , "formal public occasion" , "Ritual - my encore (anag)" .)