One formally agreeing witches meeting aren't bad (10)
I believe the answer is:
covenanter
'one formally agreeing' is the definition.
(someone agreeing to a covenant)
'witches meeting aren't bad' is the wordplay.
'witches' becomes 'coven' (a coven is a group of witches).
'meeting' is a charade indicator (letters next to each other).
'bad' indicates an anagram.
'arent' is an anagram of 'anter'.
'coven'+'anter'='COVENANTER'
(Other definitions for covenanter that I've seen before include "Principled Scotsman" , "17th century fighter for the Kirk" , "Scottish Presbyterian" .)