A Catholic curse about claimant (7)
I believe the answer is:
accuser
'claimant' is the definition.
(synonyms)
'a catholic curse about' is the wordplay.
'catholic' becomes 'c' (abbreviation as in RC for Roman Catholic).
'about' indicates an anagram.
'curse' anagrammed gives 'cuser'.
'a'+'c'+'cuser'='ACCUSER'
(Other definitions for accuser that I've seen before include "Zola, for example" , "One who brings a charge" , "Person making a charge" , "One making allegation" , "one levelling the charges" .)