Large part of church falling away (5)
I believe the answer is:
lapse
'falling away' is the definition.
(I've seen this before)
'large part of church' is the wordplay.
'large' becomes 'l'.
'part of church' becomes 'apse' ('apse' is part of 'church').
'l'+'apse'='LAPSE'
(Other definitions for lapse that I've seen before include "Slip back" , "Temporary drop in standard" , "Negligence" , "Loss (in concentration, say?)" , "Decline into an inferior state" .)