Discharge rector replacing bishop as head of trust (7)
I believe the answer is:
relieve
'discharge' is the definition.
(I know that relieve is a more specific form of the action discharge)
I can't explain the remainder of the clue.
(Other definitions for relieve that I've seen before include "Soothe" , "Make (pain) less severe" , "Break the sameness of" , "Assuage - take over duty from" , "Palliate" .)