Seeing that transgression is supported by Church (5)
I believe the answer is:
since
'seeing that' is the definition.
(both can mean 'when considering...')
'transgression is supported by church' is the wordplay.
'transgression' becomes 'sin' (sin is a kind of transgression).
'is supported by' says to put letters next to each other (in a down clue, some letters hold up others).
'church' becomes 'CE' (abbreviation for Church of England).
'sin'+'ce'='SINCE'
(Other definitions for since that I've seen before include "in the past" , "Because; ago" , "From that time one" , "Seeing that" .)