Mockery of Catholic, one involved in high mass (7)
I believe the answer is:
sarcasm
'mockery' is the definition.
(I've seen this before)
'catholic one involved in high mass' is the wordplay.
'catholic' becomes 'rc' (Roman Catholic).
'one' becomes 'a' (a thing is one thing).
'involved in' is an insertion indicator.
'high' indicates an anagram (like drunk).
'rc'+'a'='rca'
'mass' with letters rearranged gives 'sasm'.
'rca' put within 'sasm' is 'SARCASM'.
'of' acts as a link.
(Other definitions for sarcasm that I've seen before include "Caustic remark" , "Cutting wit" , "Mocking irony" , "Lowest form of wit?" , "Scornful jibe" .)