Large part of church in decline (5)
I believe the answer is:
lapse
'decline' is the definition.
(I know that lapse is a more specific form of the action drop away)
'large part of church' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'part of church' becomes 'apse' (part of a church building).
'l'+'apse'='LAPSE'
'in' acts as a link.
(Other definitions for lapse that I've seen before include "Fall from grace" , "Brief (memory) failure" , "Error; oversight" , "Slight error as caused by inattention" , "Error; expire" .)