As religious leader, taking on bishopric in good faith (9)
I believe the answer is:
sincerely
'in good faith' is the definition.
(synonyms)
'as religious leader taking on bishopric' is the wordplay.
'as' becomes 'since' (both can mean 'because').
'leader' says to take the initial letters.
'taking on' says to put letters next to each other.
'bishopric' becomes 'Ely'.
The initial letter of 'religious' is 'r'.
'since'+'r'+'ely'='SINCERELY'
(Other definitions for sincerely that I've seen before include "word often preceding a signature" , "With genuine feeling" , "In a genuine way" , "Really" , "In good faith" .)