Left part of church in decline (5)
I believe the answer is:
lapse
'decline' is the definition.
(I've seen this before)
'left part of church' is the wordplay.
'left' becomes 'L' (common abbreviation).
'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 "Moral fall" , "Pass into disuse" , "Fall from usual standard" , "moral shortcoming" , "falling away" .)