Repent, possibly, before church reveals sham (8)
I believe the answer is:
pretence
'sham' is the definition.
(thesaurus)
'repent possibly before church' is the wordplay.
'possibly' indicates anagramming the letters.
'before' says to put letters next to each other.
'church' becomes 'ce' (abbreviation for Church of England).
'repent' with letters rearranged gives 'preten'.
'preten'+'ce'='PRETENCE'
'reveals' acts as a link.
(Other definitions for pretence that I've seen before include "Make-believe, perhaps a phoney claim to status (8)" , "Ten creep (anag.)" , "Affectation" , "it can't be true!" , "Make-believe, or false claim to merit" .)