Because of transgression, going to church (5)
I believe the answer is:
since
'because' is the definition.
('since' can mean 'because')
'transgression going to church' is the wordplay.
'transgression' becomes 'sin' (sin is a kind of transgression).
'going to' is a charade indicator (letters next to each other).
'church' becomes 'CE' (abbreviation for Church of England).
'sin'+'ce'='SINCE'
'of' is the link.
(Other definitions for since that I've seen before include "Because; ago" , "in the past" , "From that time one" , "Seeing that" .)