This is the ultimate in blasphemy? (6)
I believe the answer is:
heresy
'blasphemy?' is the definition.
(I've seen this before)
'this is the ultimate' is the wordplay.
'this is' becomes 'heres' (both can be used to point out someone or something).
'the ultimate' becomes 'y' (I can't explain this - if you can you should give a lot more credence to this answer).
'heres'+'y'='HERESY'
'in' is the link.
(Other definitions for heresy that I've seen before include "refusal to conform" , "Unorthodox religious opinion" , "Unconventional belief" , "unorthodox beliefs" , "Opinion contrary to accepted religious belief" .)