Liberal section of church backsliding (5)
I believe the answer is:
lapse
'backsliding' is the definition.
('lapse' can be a synonym of 'backsliding')
'liberal section of church' is the wordplay.
'liberal' becomes 'l' (abbreviation).
'section of church' becomes 'apse' (I've seen this before).
'l'+'apse'='LAPSE'
(Other definitions for lapse that I've seen before include "Pales into omission" , "Untypical mistake" , "Moral fall" , "Brief failure (of concentration)" , "Brief failure of judgement" .)