I’d backed priest, one controlling everything (8)
I believe the answer is:
director
'one controlling everything' is the definition.
(eg someone in charge of a business)
'i'd backed priest' is the wordplay.
'backed' shows that the letters should be reversed in order.
'priest' becomes 'rector' (I've seen this before).
'id' in reverse letter order is 'di'.
'di'+'rector'='DIRECTOR'
(Other definitions for director that I've seen before include "He guides and shapes a play or film" , "One who controls resources and expenditures" , "Manager, governor" , "leader" , "Company board member" .)