Repent, miserably, before church reveals sham (8)
I believe the answer is:
pretence
'sham' is the definition.
(thesaurus)
'repent miserably before church' is the wordplay.
'miserably' is an anagram indicator (I've seen this in other clues).
'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)" , "Erect pen (anag) - charade" , "Acting of making something appear true when it's not" , "Sham" , "Deceit, dissimulation" .)