Necromancer’s errors unexpectedly overwhelming church (8)
I believe the answer is:
sorcerer
'necromancer's' is the definition.
(synonyms)
'errors unexpectedly overwhelming church' is the wordplay.
'unexpectedly' indicates anagramming the letters.
'overwhelming' means one lot of letters goes inside another.
'church' becomes 'CE' (abbreviation for Church of England).
'errors' is an anagram of 'sorrer'.
'sorrer' going around 'ce' is 'SORCERER'.
(Other definitions for sorcerer that I've seen before include "Potter, say" , "Necromancer or wizard" , "Magician aided by evil spirits" , "In stories, he has magical powers that he uses to harm others" , "Magician with musical apprentice" .)