Catholic about to practise set of beliefs (5)
I believe the answer is:
credo
'set of beliefs' is the definition.
(I've seen this before)
'catholic about to practise' is the wordplay.
'catholic' becomes 'C' (abbreviation as in RC for Roman Catholic).
'about' becomes 're' ('re' means regarding or about).
'to practise' becomes 'do' (synonyms).
'c'+'re'+'do'='CREDO'
(Other definitions for credo that I've seen before include "Part of the service" , "Section of mass" , "music in church" , "Set of beliefs about leftist circle" , "Doctrine or belief" .)