Genuine evil about to engulf church (7)
I believe the answer is:
'genuine' is the definition.
('sincere' can be a synonym of 'genuine')
'evil about to engulf church' is the wordplay.
'evil' becomes 'sin' (I've seen this before).
'about' becomes 're' (regarding**).
'to engulf' indicates putting letters inside.
'church' becomes 'ce' (abbreviation for Church of England).
'sinre' enclosing 'ce' is 'SINCERE'.
(Other definitions for sincere that I've seen before include "Free from deception" , "Free from pretence" , "Lacking in deceit" , "Perfect" , "Earnest" .)