Mission of editor reversing what predecessor left in place? (8)
I believe the answer is:
delegacy
'mission' is the definition.
(synonyms)
'editor reversing what predecessor left in place?' is the wordplay.
'editor' becomes 'ed'.
'reversing' says the letters should be written in reverse.
'what predecessor left in place?' becomes 'legacy' (the legacy left by a predecessor).
'ed' backwards is 'de'.
'de'+'legacy'='DELEGACY'
'of' acts as a link.
(Other definitions for delegacy that I've seen before include "Eg a Clyde (anag) - giving authority to others - university department" , "Committee" .)