Section of sacred opus? (5)
I believe the answer is:
credo
'section' is the definition.
(I have seen 'Section of mass ' mean 'credo' so perhaps 'section' could also mean 'credo')
'of sacred opus?' is the wordplay.
'of' indicates the answer is hidden within the clue.
'CREDO' can be found hidden inside 'sacred opus'.
(Other definitions for credo that I've seen before include "mission statement" , "Doctrine or belief" , "System of principles" , "Apostles' beliefs" , "Formal statement of belief" .)