Avoiding extremes, witches reformed moral system (5)
I believe the answer is:
ethic
'moral system' is the definition.
(I've seen this before)
'avoiding extremes witches reformed' is the wordplay.
'avoiding extremes' means to look at the middle letters (letters at the extremes of the word are removed).
'reformed' indicates an anagram.
The central letters of 'witches' are 'itche'.
'itche' anagrammed gives 'ETHIC'.
(Other definitions for ethic that I've seen before include "Ideology" , "Set of moral principles" , "Moral practice" , "Moral value" , "type of behaviour" .)